Rather than asking other people if they're neurodivergent, I normally just share my own ADHD diagnosis and see if they reciprocate.

Some people can be "coded" autistic without actually being autistic - for example, there's a strong link between the genes responsible for (inherited) epilepsy and those linked to autism, and have occasionally assumed that someone is autistic but they have epilepsy instead. (They have just two or three characteristically autistic traits.)

Some people, of course, simply have no idea.

The other simplest litmus test is to see their first instinctive, prejudiced reaction towards you.

On first meeting someone, within the first ten seconds, most neurotypical people decide that they don't like me much. It doesn't matter how I dress or what I say, they can sense on a primal level that I'm different from them and they find it unsettling and try to limit the time they spend with me.

It's been that way since I was four.

By contrast, neurodivergent people instantly, very strongly, decide that they like me.

A goth got on the bus once and ignored all the other empty seats and sat next to me because, even though I was a fortysomething woman in ordinary clothes, it's like he could sense on some primal level that he'd be safe with me.

On a training course once, the instructor paired me with the other ADHDer (years before my diagnosis) - he understood within the first minute of the session that I would get on very well with the other person, and we remain friends to this day.

Or, as the meme goes, "If all of your friends are neurodivergent and you're not, I've got news for you, buddy ..."