Check to see if she looks at you a certain way.
This specific way tends to involve blushing, looking away, or eyes that seem to sparkle. If you are having a normal conversation with her, but her eyes seem to be wide and her pupils are big, she may have romantic feelings for you. The pupils of the human eye dilate when a person sees someone to whom she is attracted.
Notice if he talks about hanging out exclusively.
If he is hanging out with you and a bunch of your friends but keeps trying to get to hang out with you alone, he probably has a crush on you. He wants to get to know you and be able to enter into a more intimate time with you without people being around. Just because he does not directly call it a date does not mean that it is not a date if he wants to be with you alone.
See if she talks a lot around you.
If she seems to talk a lot when she is around you but does not talk around others as much, she may be trying to keep you around or get attention from you. This could be a sign that she has feelings for you but does not know how to express them. Notice how she acts around others; if she seems to keep talking whenever she sees you, she may be interested.
Observe his body language.
Body language is a giveaway because it generally cannot be controlled, since it is subconscious. If a guy is in a group but he is consistently facing you with his body, you may be where his thoughts are. If his arms are crossed whenever he talks to you, he may not be interested in you, but if his arms are open and he keeps smiling at you, it may be revealing of a crush.
Listen to the questions she asks.
Does she want to know if that girl you were with is your girlfriend? Has she asked you about your past? If she keeps bringing up questions that have to do with your dating status or your type of girl, she may like you. Furthermore, she may do her best to ask in a subtle way, but if the questions all have to do with if you are taken or not, her intentions are clear.