How to know if your boyfriend likes another girl?

Ok well my boyfriend and i have been together 9 months and lately i have been noticing his strange and different behaviour around a certain girl who happens to be in his class. I always see the two of them together and they both act very flirtatious around each other, like touching each other and constantly mucking around together. however my boyfriend and i spend tinme together after school and lately he puts this other girl before me hangs out with her and not me and blows of our previous arrangements for her. i undersatnd that he can have other friends who are girls and i'm fine with that it just seems lately that maybe he has feelings for this other girl. I know that this girl likes him and its obvious everytime i'm around him she comes up buts in and just starts flirting with him infornt of me. anyway so do you think my boyfriend likes this other girl? and what should i do about it? thanks :)