Another thing I want to mention is, over the years I have asked many, many people parking in handicapped-accessible parking if they did in fact have a permit to park there. On a few occassions I have been shown a valid placard, which was poorly displayed or not yet put up (or just put away) by the driver. When this happens, I always am very apologetic and explain why I asked them about their permit in the first place (the glut of abuse, the need that those who truly are disabled have for those spaces, etc).
What I have come to notice, by and large, is that those who are legitimately parked in the handicapped spots are very understanding and empathetic to my questioning about their lack of a visible permit. They understand because they often deal with the same hardships that I (and other handicapped individuals) must endure associated with people abusing those spots.