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If he's already looking at you, he has taken interest in you. This method works better in room full of people. If he is just as flirtatious and friendly, he may be playing the field a bit. If he is clearly reserving something special for you, it's a direct hit. For example, pretend to fall over and act a little upset about it. If he is at your shoulder for the rest of the day, this proves he may be interested in you. If he teases back, tease him more. Playful bantering often occurs between lovers, as this is a way to cause laughter and happiness. If he immediately smiles back, this is another sign that he might like you. If he asks questions about your reactions to certain actions (ex: "How would you react if I did [insert action] to you?"), there's a possibility that he wants to know how you'll react to him doing something a little more personal with you. Usually best friends tell their friends who their crushes are. If he doesn't tell you or acts embarrassed, he may not want to admit his feelings to you. Keep acting innocent, and try to persuade him to tell you. For example, say something along the lines of, "Please? We're best friends. You know I won't tell anyone." However, avoid pushing him too hard, or he may grow uncomfortable. Talk to him in private to see if he confesses his feelings for you.
Briefly glance at him every so often. See how he acts around other girls. Intentionally do something that would be potentially upsetting to you. Tease him. Smile at him. Talk about who you like (assuming that you like this guy/girl too). Ask him who he likes. If you do like him, try reversing the strategies listed to show you like him. Tell him that you might be interested in him.