How do you know someone has a PhD?

When it becomes acutely relevant, they'll politely let you know, and then you can become annoyed at them about it.

Thinking about it, that exact thing also applies to other 'how do you know someone is/has/does [...]' as well.

Funny enough, my boss has a PhD in Evolutionary Biology. She never tells people because they start referring to her as Doctor, and she hates that. I don't think I've actually ever heard her bring it up on her own.

Sometimes they don't tell you and just quietly update all of their usernames...

I mean yeah, if you spent 5 years of your life pushing the edge of human understanding on a subject, and a shithead tells you to do the science on your research subject, it's relevant lol

Tbf, they kinda earned the right to brag.

Well you don't know people with PhD that don't tell you they have one

This is putting confirmation bias to the extreme.

I never tell people I have a PhD. It's rude, plus I don't have one.

True, but I do think it was warranted in this case.