Not sure if you're being sarcastic or not, but just to air this out, there are tons of things we as pilots are required by law to self ground for that would be transparent to thousands of other professions including many recreational activities. If you call in sick with a broken leg and are caught climbing Mt Everest, I guess they got you dead to rights. But you can drink beer with a headache or the squirts, paint the house with an upset stomach and even waterski with pink eye. Only one of those four things can be verified by a doctor most of the time even if they are all true.

No pilot, especially for a top tier safety first airline, should ever have to think twice about calling out for a legitimate illness, even if the condition in question is hard or impossible for a third party to "prove". To get to that point, you are going to get some blowback from a certain amount of dishonest behavior from some people from time to time. And that sucks. But it beats the alternative of even a small number of pilots potentially flying sick because they are worried about verifiability or discipline.