Who better to tell you to know when a guy is attracted to you or not than a GUY.
If a guy is attracted to you he will do very common things, you just need to look for these things that he might do. The most common things are texting, but watch out for obsessiveness - that means he's probably desperate. Then there's the old what are you doing this weekend? He's most likely wanting to know where you will be and some way make sure he will be there to, or if your not busy he might then ask you to hang out with him. Don't be fooled by a meeting for coffee or a trip to the movies, he's obviously not just looking to have a buddy if he wants to hang out with you alone. Just make sure that these meetings are about the two of you. Also make sure he's not married or already has a girlfriend, unless your in to that sort of thing.
If you like a guy and want to know if he is attracted to you then you need to ask him questions. If that doesn't work you need him to ask you questions. Use body language and touch him in some way that is appropriate. I always like it when a girl comes up behind me and puts her hand on my back, just to let me know she is there. Come up with a funny handshake, get him to touch you without being sexual.
If he has never seen you dressed up, then do you best one day and go over the top, dress for success. Look as sexy as you can and see what he does or says. He might say that you look nice, or damn you look hot. Think about how he reacts and then determine whether or not he is attracted to you. For most of us guys we are attracted to what is attracted to them assuming they are not gay.
Show off your assets, guys are terrible at staring, if you catch them looking let them know. This will be a good indication that they are attracted. If you don't catch them staring but rather looking away or not showing much interest in looking directly at you they are either shy or simply not attracted. So if its the shy guy, then you will have to make the first move. He might be a lion deep down inside but you make him weak in the knees. Life is short, if you really want to know, just ask him. He might give you a lie, but I would probably say to a girl I wasn't attracted to that first of all I have a girlfriend, and second I think your a good looking girl but just not my type.