How to differentiate between a spiritual experience and a mental disorder or delusion?

It is a good question you ask but there is a certain sector that might Blur the line between spiritual experience and a disorder.

Here is an ancient story of a real monk...

Once there was a monk in srilanka (about 2000 years ago) he used to practice "Attika Sanna". It is a form of meditation that focus on the bone structure of a being.He was on his way from "Anuradhapura" ancient city where he met with a beautiful woman. She was actually going to her parents place after a fight with her husband. She saw the monk and smiled. But the monk did not saw her as a person, He only saw her skeleton. Her husband was coming to take her back home but he was far behind. The monk went on his way and met the husband on the way, He asked "did you saw a woman going this way?". The monk replied "I didn't saw any woman but i saw a skeleton walking that way.".

It is good to take a look at what a disorder is...

Mental illness refers to a wide range of mental health conditions — disorders that affect your mood, thinking and behavior. Examples of mental illness include depression, anxiety disorders, schizophrenia, eating disorders and addictive behaviors. Many people have mental health concerns from time to time.

What science see as a disorder is something that is a bit far from what is normally accepted.But as Buddhists we try to change our mindset with meditation.It actually changes the brain structure.

Mindfulness Can Literally Change Your Brain

So a changed brain will not work like a normal one. It will most certainly behave in different set of rules.This would justify stories like the one above.

So how to recognize something gone wrong?

Buddhism's advice on that is to have a well experienced teacher who can understand and help. I must say that this person must be a Monk because a lot of times people who advice their friends on meditation get carried away by their ego and give misguided advice.

In the book Saints and Madmen, David Lukov puts it this way - "The mystic swims in the same waters where the madman drowns." I think that's a pretty good barometer. When the types of things you mention happen to people engaged in spiritual practice, there is [usually] a sense that they have some sort of control over the experience. At the very least, they know that what they are doing is the product of their practice and can be attributed to rational cause. For the insane, I don't think there is the same sense of control or causality.

Conventionally, the people who are concerned (for example a family member or doctor) may consider a mental state to be "disordered" if:

• It's persistent
• You're unable (not to mention "unwilling") to behave conventionally or skillfully

Wikipedia - Mental disorder (definition) says,

For a mental state to classify as a disorder, it generally needs to cause dysfunction.

According to DSM-IV, a mental disorder is a psychological syndrome or pattern, which occurs in an individual, and causes distress via a painful symptom or disability, or increases the risk of death, pain, or disability; however it excludes normal responses such as grief from loss of a loved one, and also excludes deviant behavior for political, religious, or societal reasons not arising from a dysfunction in the individual.[

So, for example, if instead you continue to ...

• Have friends
• Talk lucidly
• Obey the law
• Do your work (e.g. pay your bills)
• Take normal care of your physical health

... then you're less likely to be considered as having a "mental disorder".

I think that this description of the 'two truths' doctrine implies that the two extremes, "conventional reality" and "ultimate reality" (which can seem like a dichotomy) might be better understood as "skillful means" and "wisdom" ... i.e. ideally a person might be able to manifest either and/or both (not just one or the other).

Giving too much importance or attachment to any experience either actual or delusional will lead to metal instability and perhaps delusion. So if you experience something then congratulations, this is great, but leaving it aside continue your practice. Reason being distinguishing real and delusional experiences is very difficult can can be futile in some instance, hence not worth giving too much importance to. I would personally err in the side of caution can consider many of such experiences as just mind playing tricks. When you stop paying attention after some time then these experiences may pass away. Best is you continue the practice giving less importance of trying not to read into too much on what you experience.

If you suffer and make others suffer, that's a disorder. If you are happy and make others happy, that's a good result. Although sometimes it has to get worse before it can get better.