They’re trained professionals who see kids with autism day in day out. They see masking all the time and they know what they’re looking for - the assessments are always carried out by people the child doesn’t know and they still manage to diagnose them. My child masks and is not obviously autistic to ordinary people in day to day interaction and was still fairly quickly diagnosed by professionals who knew what they were looking for and put him in situations that showed it.
To my knowledge, and certainly in my experience, it is not one person’s opinion - during the ADOS my son saw a speech and language therapist and some other professional I can’t remember for a “play” session, and we had an interview with a paediatrician. Those people, plus possibly others, had a meeting and looked at the outcomes of all that, plus school evidence, plus a specialist teacher observation as well before a whole team agreed his diagnosis. Even if your son doesn’t have that many people involved it still won’t be just one opinion.
My advice is to take in a list or timeline of things you’re concerned about, and anything that’ll help you remember when he did what as a baby and toddler - I found they wanted a lot of detail about his early milestones. And prepare your child for the process - not the activities themselves, but the idea that they’re going to go into a room with two strangers to play without me needed a lot of talking through with my DC in advance.
Where you are now was the hardest part for me. Once we all (us, DC, his teachers) knew he was autistic and how best to help him he has absolutely thrived. But those months of filling out paperwork and rehashing with loads of professionals all the things he couldn’t do, all the ways his behaviour was difficult and all his social struggles were absolutely heartbreaking and awful, I sympathise. It’s natural to find these assessments really stressful, but I can say for us it all worked out and was worth it.