How to find the right Dhamma teacher

Be wary of the concept of guru and especially the person who claims to be one. Extraordinary teachers are rare and impossible to find. It is better to find a traditional and experienced teacher who makes no special claims. You may even consider a modern meditation center that offers MBSR, MBCT, and psychotherapy that has a large membership. Progress in meditation is largely psychological.

Sati,

assuming that a seek "for a right Dhamma-teacher" underlies here by the desire to gain Awakening and answering the question of what a seeker might possible do, for finding a proper teacher, is very good given in Bhante Thanissaro's Essay:

The Power of Judgment, by Thanissaro Bhikkhu (2011; 6pp./19KB) Developing good judgment is the key to becoming a true friend — first to yourself and then to those around you.

All of the possibility to gain such a teacher is by the seeker at least, or let my person say be being touched, good or bad, comes it's way.

How it happens, that a person gain Dhamma, enters the stream, is sure best explained by the Buddha himself as reported:

[Kapadika Bharadvaja:] "To what extent is there an awakening to the truth? To what extent does one awaken to the truth? We ask Master Gotama about awakening to the truth."

[The Buddha:] "There is the case, Bharadvaja, where a monk lives in dependence on a certain village or town. Then a householder or householder's son goes to him and observes him with regard to three mental qualities — qualities based on greed, qualities based on aversion, qualities based on delusion...

This Sutta and good sorted others, as well as practical advices, Orion may find in the given essey:

Into the Stream - A Study Guide on the First Stage of Awakening

[Note: This is a gift of Dhamma, not meant for commercial purpose or other gain in the world]

I tried to follow everything from the nissayamuccaka-course which I described in this answer, when I was still in a monk-hood by myself; and I had a low-quality teacher for about five years; before I found a perfect teacher from pa-auk forest monastery (this monastery also published other English-language Dharma articles here) who have all of the qualities listed below, that were specified in the tipitaka and commentary.

I met the pa-auk teacher too late, so I am being a layman now. However, it was my previous effort and experience which gave me the confidence to choose pa-auk to be my kammaṭṭhāna teacher.

The qualities of a teacher, and the operating procedure to choose and to apply with the teacher, have been briefly described in Chapter 4 of the Path of Purification.

Furthermore, in my opinion, the significant quality of a teacher is paṭisambhidā: because all ariya must have paṭisambhidā, at least in his kammaṭṭhāna. So he should have these qualities:

1. His teacher descent, he must derived from a tipitaka memorizer (buddha-sāvaka, suta-buddha; V.N. Mahāvagga Nissayamuccnakathā).
2. His enlightenment, he must achieved at least upacāra-samādhi and balava-vipassanā (4 paṭisambhidā; V.N. Mahāvagga Nissayamuccnakathā).
3. His tipitaka memory, he must graduated through at least nissayamuccaka-course (V.N. Pācittiyakaṇḍa Ovādakasikkhāpada's commentary).
4. His tipitaka understanding, he must not cut tipitaka off or make tipitaka conflict each other. Because tipitaka was memorized by single commentary teacher group at 1st saṅgāyanā, so it must be a single package that compatibility with each other (pro in tipitaka relation, dhamma-paṭisambhida&attha-paṭisambhidā).
5. His pali skill, he must understand pali in advance (nirutti-paṭisambhidā).
6. He must be a genius one (paṭibhāna-paṭisambhidā).

Furthermore, in tipitaka and commentary tradition, since mahākhandhaka of vinaya mahāvagga is completed authored by upāli in 1st saṇgāyanā, all layman's teachers must have all of these qualities, that concluded by upāli in bhikkhunovādakasikkhāpada's commentary (which I translate below as follows):

Qualities of nissayamuccaka-bhikkhu (teaching lay people)

1. Proficient to recite pāṭimokkha-pāli and to understand it's commentary.
2. Proficient to recite and to understand 4 bhāṇavāra (~1,000 syllable) of sutta and their commentary, to teach laymen on uposatha day.
3. Proficient to recite and to understand sutta for bhikkhu's life such as andhakavindasutta, mahālahulovādasutta, ambaṭṭhasutta, etc.
4. Proficient to recite and to understand sutta for teaching in 3 chances: banquet for saṅgha by layman (nidhikaṇdasutta), funeral ceremony (tirokuṭṭasutta), and auspicious ceremony (maṅgalasutta).
5. Enough understand to judge/to decide about saṇgha's ceremony such as uposatha, pavāraṇā, etc.
6. Proficient to recite and to understand his kammaṭṭhānā throughout the nibbāna-course.
7. 5 years experience in monk hood as a monk.

Qualities of bhikkuparisūpaṭṭhāpaka-bhikkhu (teaching bikkhus)

If above layman's teachers want to teach bhikkhus (ūpajjhā-ācāriya, nissaya-ācāriya), they must increase their skill level to all of the following qualities.

These are for abhivinaya teaching:

1. Proficient to recite mahāvibhagha and bhikkhunivibhaṅga (first 3 books of thai 45 books pali-tipitaka) of vinaya-pitaka-pali. At least, he can relay with the other 3 bhikkhu. Proficient to understand it's commentary, too.
2. Proficient to recite all saṇgha's ceremony in vinaya-pitaka mahāvagga and julavagga.
3. Proficient to recite 14 vatta in vattakhandhaka.

These are for abhidhamma (kammaṭṭhāna) teaching:

4. Proficient to recite one of this suttanta-pali: mūlapaṇṇassa (1st/3 parts of M.N.) for student in M.N. faculty, mahāvagga (2nd/3 parts of D.N.) for student in D.N. faculty, sagāthavagga+nidānavagga+khandhavāravagga of S.N. or mahāvagga of S.N. for student in S.N. faculty, before half of A.N. or after half of A.N. or ekakanipāta+dukanipāta of A.N. for student in A.N. faculty, jātaka+commentary (because kammaṭṭhāna was described in commentary) for student in jātaka faculty.

Qualities of bhikkunovdaka-bhikkhu (teaching bhikkunīs)

If above layman's teachers want to teach bhikkhunī, they must increase their skill level to all of these qualities:

1. Proficient to recite whole tipitaka-pali and commentary-pali. Or at least, he still must recite whole tipitaka, but he can recite just one commentary of suttanta, first 4 parts of commentary of 7 parts of abhidhamma. However, vinaya-commentary is what he must recite it all.

Reference: tipitaka and commentary of vinaya pācittiyakaṇḍa bhikkhunovādakasikkhāpada and vinaya mahāvagga mahākhandhaka.

(I still find it difficult to reference the tipitaka and commentary using english resources: there's no translation, very long pages, confused categories, cutting some parts off the tipitaka, cutting commentary off from pali canon, etc.)

Related topic: Should Lay Buddhists Teach the Dhamma?

The Udayi Sutta (AN 5.159) describes the five qualities in one qualified to teach the Dhamma:

"It's not easy to teach the Dhamma to others, Ananda. The Dhamma should be taught to others only when five qualities are established within the person teaching. Which five?

"(1) The Dhamma should be taught with the thought, 'I will speak step-by-step.'

"(2) The Dhamma should be taught with the thought, 'I will speak explaining the sequence [of cause & effect].'

"(3) The Dhamma should be taught with the thought, 'I will speak out of compassion.'

"(4) The Dhamma should be taught with the thought, 'I will speak not for the purpose of material reward.'

"(5) The Dhamma should be taught with the thought, 'I will speak without hurting myself or others.'

Note: According to the Commentary, "hurting oneself" means exalting oneself. "Hurting others" means putting other people down.

And how do you know if the teacher teaches the right Dhamma? Use the criteria of the Kalama Sutta as mentioned in Andrei Volkov's answer.

I would recommend you to pick the tradition closest to the original teachings of the Buddha and try to find a good teacher within that tradition. Trying many traditions would probably lead you to spiritual confusion than progress.

You can discover a good Dhamma teacher by evaluating the following criteria

1. Are his teachings sensible, skillful and praised by the wise?
2. Is he living according to what he preaches?
3. Are the teachings inline with the Tipitaka and the commentaries?
4. Does your understanding of the Four Noble Truths increase by following his teachings?

The Dhamma of the Buddha is the right dhamma teacher; as stated in the Pali suttas.

I have set forth the Dhamma without making any distinction of esoteric and exoteric doctrine; there is nothing, Ananda, with regard to the teachings that the Tathagata holds to the last with the closed fist of a teacher who keeps some things back... Therefore, Ananda, be islands unto yourselves, refuges unto yourselves, seeking no external refuge; with the Dhamma as your island, the Dhamma as your refuge, seeking no other refuge. DN 16