To use handicapped parking in Maryland you must either have a metal tag or temporary handicapped placard issued to you. Doctors documentation is required for either type of media. Placards can be taken with you when traveling out of state. If documentation is forged or found to be false the person requesting the tag is prosecuted. If a doctor is issuing a statement falsely, they are prosecuted and barred from providing documentation as there are control numbers assigned to doctors. That takes care of parking only. And I know because I work at the DMV in Maryland.
Now as to the people faking it. Make medical documentation necessary for getting a GAC card. This does not affect the ADA compliance issues. Anyone that needs the GAC will have no issue getting the documentation for ECV or line bypass.
I have extreme photosensitivity. Translated to people talk that means I am sensitive to bright light and am required to wear dark glasses when in bright light such as sunlight or brightly lit rooms. As such, when I had to apply for my US passport, I had to include a letter that explained the condition as well as photos of me with the glasses on. As this photo is on my passport, it is official US issued ID, I never can be asked to remove my glasses by anyone for any reason. However, I can if asked stand light for a very brief time, and it has been 6 years since the onset, but I have yet to be asked. My recent trip to Jamaica was uneventful pertaining to my vision issues. IN fact it was met with a lot of curiosity by people in TSA and people in Jamaica. I am not standoffish, and it has taught me more about how to converse with people.
So, as I am defined as disabled under the ADA, the fact that people take advantage of systems put into place to help disabled people, offends me to no end.