In order to get a parking pass you have to go to the doctor and then fills out a form you send it to the state and you are issue a parking placement card and a card .
To which I would answer this: In NY, once you get the handicap sign to put over your rear view mirror, it can be given to ANYONE to use/abuse. Years ago, I rode the train from Long Island to my job in NYC (1 hr + ride). Every morning, there was this brand new Corvette in a handicapped spot right near the stairs to get to the platform. One morning, I happened to see who got out - a young woman, about 25. She got out of the car and sprinted up to the train and hopped right on. As I walked past the car, I happened to see her roller blades in the car on the seat. So....my thought was Grandma/Grandpa got the pass, gave it to darling granddaughter so her shiny new car was in a separate location away from the masses and also so close she needn't worry about getting wet/cold/wind damaged.... Could she have had a disability that I didn't see - sure, but I will bet anything it was a scam....
Now, to the Disney relevance -
Over the years I have travelled with many different friends. A few times we have gotten a GAC. One was for a friend who had recently completed radiation therapy for prostate cancer. From all outward appearances, he looked healthy. But the radiation caused weakness in his legs and he found it hard to stand for long periods. So, we used the card when we could so he could enjoy more of his vacation. There were rides we didn't need it on, and there were times he went back to the room to rest and we did not take the card from him.
I have another friend who has systemic lupus - again, you would not know it by looking at her. But if you could see the scars from the pick line for IV treatments every night, you would see she has battled fiercely. She will make use of the GAC to minimize her wait times when possible because she knows she will have a limited amount of energy and these vacations are important to her. She often does not want to use a wheelchair, but we have insisted so that she can reserve what energy she does have for the things she wants to do. When we get to any ride, we always say she can ambulate and we will go throug the cue as necessary.
In both of these cases, the GAC helped make the guest's experiences better by allowing them to enjoy more of the park within the scope of their disability. I have never judged anyone who uses a GAC since you simply 'don't know' by looking. But I have seen people who will trade it off, use a wheelchair to bypass part of the line and scam their way through the day. If you challenge them, most likely you will get the same response as those who would hoard fastpasses all day and then use them anytime they wanted, ie "It's something that Disney hasn't enforced, so why shouldn't I take advantage of it."
My solution - have various versions of the GAC. Autistic child who can't wait in lines? Blue card - use alternative entrance for guest and reasonable party. Broken leg/wheelchair/bone issue? Red card - use the alternative entrance. Heat/asthma/lung issue? - Yellow card - no reduction in lines, but reasonable accomodation when possible. Finally, the person needing the card must be present to acquire it, and their photo should be attached. Not a perfect solution, but might lead to discussion that will!
Sorry for the long post!