He didn't introduce you to his friends
If he really likes you and wants to date you, he will introduce you to his friends. However, if you have never met his friends, it means that he intends to let you avoid meeting his friends. You know what? It's an important rule for all hookup finders to maintain a good NSA relationship not to let their sexual partners see their friends. This means that for him, you are not his date partner, but just a FWB. When no one around him knows you exist, they can easily end the relationship at any time they want. However, as you get to know more and more of your friends, things get more complicated. This is not good for someone who may leave you at any time.
He seldom contacts you
For a person who really likes you, he will be able to think of you anytime, anywhere, and always can't help but want to contact you. However, in a FWB relationship that's only supported by sex, it's normal for your partner to not contact you for a few days. No small detail in his life reminds him of you. It's only when he wants to enjoy sex that you show up in his mind. It's a clear sign that you're just his sexual partner. Your partner doesn't care about how you feel or what happens to you every day. What he cares about more about you is just sex. So, when the person you like seldom contacts you, it means that you are not so important to him.
You make love every time you meet
There's nothing like having sex every time you meet to show that you're in a FWB relationship. For ordinary lovers, you have many reasons to meet, such as traveling together, going to a movie or going out for a meal, etc. However, you meet just for sex. This means that your life has nothing to do with each other except sex. If so, then you should be careful. If you don't want a NSA relationship, you should leave it as soon as possible.