Does an Arahant yawn? Or: What are the characteristics of the Noble Ones: Sotāpanna, Sakadāgāmi, Anāgāmi and Arahant?

A good collection of how to recognize an admirable person can be found in I: The Way to Stream Entry.

Keep in mind:

"'It's through ... ... that a person's ... may be known, and then only after a long period, not a short period; by one who is attentive, not by one who is inattentive; by one who is discerning, not by one who is not discerning': Thus it was said. And in reference to this was it said.

And one fact, seldom pointed out, Ariya puggala are free of the five kinds of stinginess, signs which help one probably most, i.e. generous people and never traders.

Feel given to get deeper into in a sphere given for such.

(Note: not given for trade, exchange, stacks and entertaining one but as a tiny door to escape)

What does the sutta's say about the characteristics of the Noble Ones? The Abhidhamma, for instance, tells me that a Sotāpanna can't break the 5 precepts anymore. This is one characteristic one could look for then.

It's from Sutta, not Abhidhamma. Sutta Pitaka Vol 10 : Sutta. Saṃ. Saḷa. pāṭaliyaSutta...

10. “Gāmaṇi, I know the destruction of living things, results for the destruction of living things, I know the method by which the destruction of living things after death lead to loss, to birth in hell.

...

43. “Here, Gāmaṇi, the noble disciple gives up and abstains from destroying the life of living things, gives up and abstains from taking the not given, gives up and abstains from sexual misbehavior, gives up and abstains from telling lies, gives up and abstains from slandering, gives up and abstains from rough talk, gives up and abstains from frivolous talk, giving up the coveting mind abstains from coveting, giving up the angry mind abstains from the defilement anger, giving up wrong view develops right view.

So, what does this all mean practically? Okay, a Sotāpanna doesn't break the precepts. What about a Sakadāgāmi? Does this elimination of grosser forms mean (s)he never has sex? Does an Arahant not yawn, because (s)he has eradicated sleepiness?

In short you can see this search result.

In detail, Sotapanna has the powerful craving in five strings and still able to angry because of those five strings, but he understood the dependent origination (right relativity of five aggregates) already, so he clearly realizes the result of all unwholesome formations. However, he can completely decide to cease only the craving in the actions which caused by wrong view, such as the five precepts because he still has the powerful craving in five strings.

Sakadagami has the weak craving in five strings and still able to angry because of those five strings. Sotapunna is just at the beginning to cease craving, but Sakadagami is at more than half of the cessation way. It's similar to a fresh man and a senior because both of them still not graduate, but they are completely different from each other because of their experience in the university. Sakadagami is seasoned.

Anagami has no sexual feeling any more. He has no craving in any five strings. He still has only the sleepiness in meditation, the forgetfulness in meditation, the craving in being, the craving in not being, and the craving in the difference of own being and the others'.

A stream-enterer has become free from the identity-view, but s/he is not free from attachment to things. There are different phases for a stream-enterer in the process of the development of the mind and the most important one is the last(end) phase. A stream-enterer can have short or long periods of bliss but the defilements would start to effect his/her mind again. That's why it's not complete enlightenment:

https://www.youtube.com/watch?v=GkmLYAl1p7w

A once-returner has continous bliss and contentment and the mind states are not seriously effecting him/her anymore. Which means neither the external world nor the internal forms(thoughts, feelings, emotions) are not damaging the person's inner peace and stability anymore. But as there is no limit of development of the mind, even this stage is imperfect in comparison of non-returner etc. But some people (outside of Buddhism) refers this stage as "attaining enlightenment". Also there are different phases in each enlightenment stages so a "10 years old once-returner" would be definitely be different than the "few days old once-returner"

https://www.youtube.com/watch?v=GRquhvbfVp4

Non-returner is the stage that sensual desire and ill-will is completely purified from a person's mind. This is a very blissfull stage. But the weak intentions of "wanting to be something" or "wanting to do something" still exists in this stage.

Arahant stage is complete and permanent purification of the mind. There is no "karmic intentions" left in a person's mind. And "not yawning" or any other physical characteristics are not necessary to exist in a fully enlightened being. Also, old habits can continue to exist even in a fully enlightened being, but that old habits would not connected to greed anger and delusion for an Arahant. Here is a speech of an Arahant:

https://www.youtube.com/watch?v=O6Isd8tSOXw

These talks about the enlightenment stages are also helpful:

Once-returner: https://www.youtube.com/watch?v=Bn-xKG2zP4w

Non-Returner: https://www.youtube.com/watch?v=1-ELXPFD0t4

I assume that you're trying to find ways to correctly identify a person's level of attainment.

The problem with this approach is that the expression of anger is proof that one is not an Arahant. However, the absence of anger is not proof that one is an Arahant. The same applies to the other levels.

In fact, "expression of X is proof that one is not an Arahant" is easily applied to an Arahant, but not easily applied to the other levels, because it gets fuzzier due to the presence of one or more fetters. A stream enterer can get angry but he would not kill anyone. But not killing someone is not proof that he is a stream enterer. Meanwhile, getting angry doesn't have any relevance to stream entry in particular.

In Udana 7.2, Ven. Sariputta (the Arahant) tried to teach Ven. Bhaddiya without realizing that he too was an Arahant.

The Buddha saw this and exclaimed the verse, describing Ven. Bhaddiya:

He has cut the cycle, has gone away to freedom from longing. The dried-up stream no longer flows. The cycle, cut, no longer turns. This, just this, is the end of stress.

This shows that the Buddha could recognize who is an Arahant, but not another Arahant.