"You know your crush is receptive to flirting if he or she is responsive," says Barbara Greenberg PhD, a clinical psychologist who specializes in family and relationship issues. "Pupils dilate when a person is interested in you. Look into the eyes of the one you are flirting with. They speak volumes."
If you're mostly limited to texting or online messaging, Dr. Greenberg confirms what many of us already know: More than likely, your crush is into you if they carry on the conversation and respond without too much of a lag time.
At a loss for the words (and emojis!) to send to someone? I put together some tried-and-true text messages to send when you want to flirt but haven't started sexting yet. (But if you're already there, here's how to kick the conversation up a notch.)