Stop making the mistake of thinking he could be treated as a normal, innocent bystander.

If George Floyd was a serial killer, and then started freaking out being clearly high on something, and then completely goes limp... for several minutes, and maybe even stops breathing, then kneeling on the back of his neck still doesn't do anything.

It neither restrains him, no asphyxiates him.

The biggest threat the officer had at that point, was from the crowd.

How could he tell that George wasn't faking it? For starters, it's a classic case of "Excited Delirium": dude is high as fuck, acting hysterical and controllable, then passing out, and dying of cardiac arrest. It's not the first time I've watched that play out in police footage, and I've even seen cops recognize the signs and realize "Oh shit, he's about to die" when the suspect basically fades out, and try CPR to get their heart started again.

Beyond that, he could check his pulse and his breathing. There's also a sternum rub and a few other tricks to try and wake people up to see if they are genuinely lights-out.