Lips
That’s right, by looking directly at someone’s lips you are giving off a clear signal that you are very much interested in them. Now, we are not saying that you should stare at their mouth for long periods of time while you sit across from them, that could just come off as creepy.
Just try looking from time to time at their lips in the middle of a conversation and they will definitely notice this. Once they see those few glances, they will subconsciously get the message that you are into them and they will have the go-ahead to make the next move.