As to the general veracity of tests, there’s a couple of things that are important.
Testing with headphones is usually less accurate than testing with inserts; peripheral leakage, external noise, differences in residual canal volume, fully collapsing the EAM etc.
Levels; 5dB steps are greater than a doubling of the sound energy (though apparent doubling is about 10dB). This might only seem like a small thing, but geometric progression actually means you’re very likely to be within one step of your actual threshold.
Fakery; yes people do try this. Poor testing technique and auto tests are subject to more errors, however once you’ve spent a few weeks doing industrial noise claims, you can spot the fakers with bells on. The lawyers hate it; they’ve usually got some time/effort invested in that client already.
It’s the simple stuff that gives it away; answering background questions from me facing the other way while I get up to close the door (with a supposed flat 70dB loss), the BC not matching the AC, reflexing to sounds they ‘can’t hear’, repeat test variability, over exertion during the test. All sorts really. I’d give them one chance to repeat the test with real values, then bounce them.
So, to answer the original question, testing is pretty reliable. The calibration of electronic kit is unlikely to wander these days (except BC vibrators) that have a habit of decaying slowly. Your Audiologist is likely to have a previous record for you; so can spot if things are ‘off’ pretty quickly.
I did have a transducer lead that had an occasional fault once; which was fun to find.