If someone is attracted to you, he is probably not going to tell you outright. Rather, he will show his attraction through a number of nonverbal cues. It is important for you to be aware of these because if you don't notice them, he (or she) will assume that you are not interested and will move on. You should also be aware of these indicators because you don't want to inadvertently give them to someone you're not interested in, as this will create an uncomfortable social situation.
Assess how close to you he is standing. If he is standing closer than most people do, then he is probably attracted to you. A good way to confirm this is to take a half step or so closer to him. If he steps backward, then he probably isn't attracted to you. If he lets you into his space, though, then he might be attracted to you.
Look at her eyes. If she's making a lot of eye contact, widening her eyes and generally focusing on you, then she is probably attracted to you. This is because eye contact is extremely intimate, and it is not a connection we make with just anyone for a sustained amount of time.
Look at his hands and check to see if he is touching his face, his hair, adjusting his clothes or generally fidgeting. This body language arises for a number of reasons. For one, if he is nervous he will fidget. What's more, he will want to look his best around you, so he will adjust his clothing and pick fluff off to ensure he is at his best.
Check her body movements against your own. If she touches her face when you touch your face, or if she takes a drink when you do, then she is mirroring you, which is a key sign of respect and attraction.
Check if he's touching you. If he finds a number of excuses to touch your arm, guide you through doors or rub something off your shoulder, then he is probably interested in you.