I'm a big flirter, its kind of part of my personality. So, Usually for me I can feel someones vibe or I see how they act around other people and how they act with me or what they say/how they say it. Also, a really good indicator if someone is interested is if you catch them looking at you quite a lot while they think you're not looking. I'm very observant lol. To flirt back: I usually say things that imply that I like them or compliment them or state things I notice about them that I like. I smile and laugh a lot but not like an obnoxious biotch. And sometimes I hold their gaze for longer than usual while smiling, not too difficult. For girls, touch is everything, not in a perverted way, just like "wow I like your bracelet, may I see?" while taking her wrist or something or touching her shoulder to tell her you're going to get something but you'll be right back. Hope that helps <3