Stevie-Ray...
... there's a big difference between asking somebody if they are handicapped, and making stage whispers about them based on assumptions.
In the OP's case, he could even ask in a semi-humorous way. Example:
"I had to give up a foot to get my pass. What did they charge you?"
Or, there's the more formal and polite manner:
"I'm sorry, but did you realize you're in a handicapped spot?"
That option allows saving of face, in case the person actually is eligible. You could even say, "Sorry, you look so healthy I didn't realize," and turn it into a sort of compliment.
But charging in and challenging could be embarrassing, or worse. More than one person got told off for approaching my friend in such manner. Had they not had the sense to leave and/or look completely embarrassed when they found out she had MS, I'd have been tempted (being 20 and having a bit of a Sicilian temper at the time) to offer to help them qualify for their own pass.
I'm older and much more mellow, now. But I still recommend a courteous approach, as it's better (and safer) for all involved.
Calling the police is also a responsible alternative option.