Your friends could be jealous and want your relationship to fail. Not saying that's the case, just saying that is one option you didn't explore in the OP
You look at the way they treat you. If they ask you do favors all the time or only talk to you when they want something, that's a good sign. Also it's good to ask certain questions to know for sure. Don't be too obvious just ask like "hey why do you only talk to me when you need something?"