Re: how tricky it is to gauge that from first impressions. Well, it can be...unless one's first impressions are based on stereotypes.

How many of my K-12 classmates exhibited homophobia against me based on my voice and mannerisms, even while I was closeted?

How many Sunday school classmates came to view me negatively because they were annoyed about how I (as a down-low polytheist) was always questioning/challenging our teacher about everything?

How many of my coworkers and social contacts have maligned my character due to misunderstandings from my autistic behavior?

Answer to all three: A LOT!

And that's just me. Survey any other array of individuals, and I guarantee you'll hear about more than just race or class (in addition to those two attributes).