One of the things I have learnt over the course of a lifetime is that most people are so preoccupied with their own appearance and self perceived shortcomings that they are oblivious to what other people look like. Obviously if there is something easily spottable, like a wheelchair or crutches etc they can tell someone is disabled. If they are looking for a blue badge space then they can be hawkeyed (and wrong) in spotting a transgressor. People drive into car parks looking keenly for a space, they notice far more than at normal times. The notion of hidden disabilities and cars for children driven by parents should change all that. In general, unless we are specifically looking we consciously notice very little. How many times do we drive home on autopilot and remember nothing?
I have seen many occasions when some fit young bloke slams into a disabled space and runs into a shop or a bank. 99% of the time they are probably abusing the system. In Sheffield at Fox valley, a butchers shop park in two spaces to sell their wares from their bespoke van. I phone the store and complain. You cant do much with young men who run in and out.