His behavior. Does he text/call you often? If you don’t answer, does he ask you what you were up too?
He makes plans. He calculates when he is going to see you next, how he’s going to act/how you’re going to act — so he can make his move.
What he talks to you about. If you just met, does he ask you how your day was? If you have know each other a while does he show interest in your hobbies, goals, thoughts, or feelings?
How he looks at you. His eyes light right up the second he sees you.
He shows no interest in other girls. He doesn’t friend a bunch of single girls on Facebook — who he may or may not actually know. He’s not texting his ex-girlfriend. He texts some of his female friends and family, that’s it — none of them as much as he texts you.