Hold eye contact, if he keeps looking then you're in.

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It's actually the same like with girls. If somebody is interested in you he keeps looking at you, sends a smile or an eye wink, makes a cheeky comment, starts a conversation, sends you a drink by the bar keeper. Then it's up to you to find out how far his interests go and if there goes more.
In cruising places where it is more about sex it might be things like playing with balls with hands in the pockets, rubbing scrotch, following you and so on. You will notice that automatically if you are looking for somebody and someone else is doing so.