I don’t think it’s a good practice to challenge anyone using a handicapped space just because he/she does not look handicapped. I agree there are some places at whcih there is a surfeit of handicapped spaces that seem never all filled. By the same token there are some locations with only 2 spaces that are almost always filled.
The only handicapped places that I've seen filled are the ones at work here, specifically I park near another building very near the handicapped places for that building and then walk a couple of hundred yards to the building in which I work. Strangely enough one of the women who work on my floor parks in one of the handicap spaces for the other building and also walks a couple of hundred yards to the my building. Plus she goes out to lunch quite frequently, and I've seen her walk to her car and back again. I'd venture to say she isn't all that handicapped, but is taking advantage of the sticker to get a reserved parking place.