You ask them.
There are no physical indicators of asexuality. We don’t all have extra ribs or pointy ears or stretchy rubber arms or glow under a UV lamp or anything like that. There’s no secret dress code for asexuals. We don’t all have black rings on our right middle fingers, black-grey-white-purple friendship bracelets, shirts that say “This is what an asexual looks like”, or ace flag bumper stickers on our cars. We don’t have a specific rhythm to our step. We don’t have a particular manner of speaking.
You can’t look at the way someone behaves, either. Asexuals act the same as everyone else. You can’t say “That person doesn’t have sex, so they must be asexual”, because being asexual doesn’t necessarily mean that someone doesn’t have sex, and not having sex doesn’t necessarily mean that someone is asexual.
The only way to know for sure if someone is actually asexual is to talk to them about it.