Have you ever trained an 8-week old puppy? If you've got the right reward - they'll do your Physics homework, but if you don't - you'll lose their attention by the time they're done chewing. Dudes are like that - so find the right reward. Could be praise, or could be holding hands, or could be letting him wear your pants - I dunno.
if u have been with this guy for a while you know how he acts and maybe even when he is lying, ask to use his phone or get on his facebook to post a status for him ask him silly questions about other girls to insinuate that he may like someone else and see if he gets offensive, i guess a better way to put it is be nosy, when a guy gets offensive or accuses you is when he is most likely doing something wrong
If you even have a question that you thinks he likes another girl then you obviously don't trust him or he hasn't made it very clear to you that he likes you. Many guys these days go out and take advantage of girls like you. Play a little hard to get and see if he takes it the wrong way. If so then he likely does like another girl and you should cut things off ASAP. There's plenty of other guys out there, and if you have the motivation to ask this question on here then you're definitely going to find a better guy out there who will better fit your needs.
if he stares at her a lot, you shouldn't be worried. Then when he day dreaming more then usually that's a sign, and finally do you see anything different about him. P.S. a big thing cheating, ditching off dates to hang out with friends or got in trouble (with some lame excuse) and had to do something to owe it.