This is truly a distasteful thread. As an observer please keep in mind:
1) it is none of your business
2) it is REALLY none of your business
3) there are many hundreds of inflammatory and neurological diseases that flare up ie Arthritis and MS. While we may go days, weeks or months without symptoms, when they appear they can be horrifically crippling and painful. Try being under 40 with an invisible disability very few people will believe you.
4) if you think someone is abusing the system call the police, they have the authority to verify the owner of a disability sticker/card. But make awful damn sure you right. Are there abusers, ya probably but should that 1 out of 100 give everyone else a bad name... no. People abuse all kinds of rules ie the teen mother on welfare living in subsidized housing and lets her working boyfriend move in, or how about that person who always takes 10, 11 or 12 items in the 8 items or less line?
5) is there grey area? yes some doctors are more liberal, some patients have higher pain thresholds, some people only have symptoms sometimes. Is that a reason to paint everyone with an invisible disability with the same brush? NO!
The point:
1) only a doctor can decide who can have a disability card, and they do not have to disclose that info to someone in the parking lot who is mad that they had to walk from the back of the lot.
2) only the police/parking enforcement can enforce the use of the cards, not someone in the parking lot who is mad that they had to walk from the back of the lot.
3) only you can not judge someone with a disability.
To the OP: suggest anyone who gives you attitude about it, in you most friendly and polite voice that they call the police if they think a law has been broken. An other option is to suggest to the person in your most friendly and polite voice that their prejudice toward people with invisible disabilities is unflattering. You would be shocked how many people with attitudes about this will shut up and crawl under a rock after being called a bigot by someone with a disability.