Usually there are no definite signs for you to know this, but the fact that you are asking this question means that deep down, you are having doubts and your gut is telling you that something isn't quite right. LISTEN TO YOUR GUT FEELING!!! Trust me, your gut knows things better than you think it does. If it is telling you that this guy is no good and causes you to question about him, then it's probably because he IS no good and that you should get away from him unless you don't mind being taken advantaged of.
Just recently, I started questioning my ex about his motivations like you are now. He said that he wanted to be friends again but he always seemed like all he wanted out of me was sex. It made me wonder if he actually wanted to be friends again or was he just trying to get someone to sleep with him? And because I was asking myself these things, I knew that something wasn't right. If things were okay, I wouldn't be doubting him or asking myself these things just like how things were when we were dating and things were fine. Of course he denied it when I asked him up front, but I couldn't get that awful gut feeling that he was just trying to use me out. That's why I decided that yeah, he was just wanting to take advantage of me and I've so far cut things off with him before I actually did anything I would regret.
So bottom line is to trust yourself. If you smell something fishy, chances are you're right! You have instincts for a reason. They are there to protect you from threats and your body is currently in questioning because it is trying to protect you from a threat (aka this guy) by making you change your mind about him. There are of course possible signs that you can try reading but they may not be accurate for every guy. That's why the number one thing you should trust when it comes to this is your own gut instinct.
