It's pretty easy to judge. My ex-girlfriend had chronic (not acute) leukemia (CML) and was in pain most of the time. Walking any kind of significant distance kicked her ass. To all of you it would just look like she was fat and lazy. She was constantly puffed up from the chemo and walked like the kid from A Christmas Story, but without the multiple layers of winter clothes. On the rare day she felt somewhat normal, we wouldn't use the HC hanger. Those days were rare.
So STFU if you don't know what's going on with a particular person. I know people are jackwagons and sometimes it's obvious they're faking, but it ain't always the case. And if you don't know, you don't know, so butt out.
(I loved being able to park up front but I know to everyone else--and I saw the stares, believe me--it looked like a couple of fatties abusing the system. F them.)