Personally, I'd say the lower 80 or 90% immediately identify themselves as such, but beyond that I try to give others the benefit of the doubt.

I'd agree with that statement, revising it up to at least 95%. Once you've got it down to more than 19 in 20 people you meet being obviously-dumb, it's worth the effort to inspect the others more carefully, since it's always good having really smart people around.

Also, I've been intuitively aware of the optimism bias for as long as I can remember, and estimates like ".01% and 99.99%" trigger my skepticism module hard.

I'm much more familiar with people thinking 95% is an orders-of-magnitude higher estimate than 80%, and so I tend to adjust others' carefully-thought-out estimates outward rather than inward, unless they are 0 or 1.

ETA: It's worth noting that one of the huge signals smart people give off is the "OMG you're talking about something that requires intelligence I'm so happy to have met a smart person because that happens to me less than 5% of the time" reaction, which if rarer than I think would significantly throw off my estimates.