@Tessasanderson
"In the current society there are not many people who will actually put themselves out for whats right and wrong. See someone getting beaten up, turn away. See someone stealing, turn away. See someone parking in a disabled space to protect their Range Rover, turn away."
The difference with the examples you give here is that they are cases where someone is clearly doing something wrong/illegal. If someone is verbally abusing or physically attacking someone else, it is an unquestionable fact that they are behaving inappropriately or breaking the law.
But - because some disabilities are invisible - it can't be assumed that a lack of visible, physically obvious disability means that the person in question is not disabled.
When a disabled person is questioned simply because they don't look "obviously" disabled - unlike the example of seeing someone get attacked - the questioner is operating off an incorrect assumption they had made, not an actual wrongdoing that they have witnessed.
They are making a presumption that the person in question is non-disabled, and given that, as stated, various disabilities are not visible, it can be argued that this is an unfair presumption to make.
I think it's better to give the benefit of the doubt, rather than potentially embarrass or upset someone with a genuine condition.
"So once in a while they get it wrong and ask someone with a not obvious disability if they are genuine. Is that such a hardship for someone actually having some social concerns?"
If the disabled person in question is a confident person, it may not be that much trouble for those individuals to be challenged - though, let's be honest, it's not pleasant for anyone to essentially be accused of lying about having a disability.
But I would imagine there are a fair few disabled people who are not confident. They may have depression, social anxiety, autism, or a whole range of conditions that could, in fact, make such an encounter very distressing for them.
So yes - for some disabled people, being challenged about their use of a space to which they are entitled could indeed be a very difficult and upsetting circumstance.
If it's done in a public place, with other people within earshot, it's also probably very embarrassing for the person being questioned.
In the absence of actual evidence that the person in question is not disabled (and not looking disabled isn't evidence) then in my opinion, it's best that other people give the benefit of the doubt and presume that the individual has a hidden disability of some sort.