It depends on the individual... But the main sign is if she likes to look at you a lot (lots of eye contact) this is a main point that she is interested in that person
Being mature enough to sense the interest another person has towards you. it is called nature. when someone likes you,, you can feel it. by the way she acts when you are around etc. i saw this question a million times and it is unbelievable cos it is so lame and stupid