How do you tell if someone is a top or bottom?

You can't "tell" from looking at them or from their mannerisms since assuming someone is a top or bottom or versatile (a different option which you did not include) based on perceived stereotypes is often wrong. Your best bet is simply to ask them. If this would seem awkward for you to do, you likely have no genuine reason to want to know anyway. Their preference is unlikely to be relevant to you unless it's likely you'll be ending up "in bed" (a euphemism) anyways. It's really up to them to decide if that's of any of your business or not. On the other hand, some people may have no qualms at all letting you know even if you're merely curious. So you'll need to ask.