Knowing weather or not you are in a relationship could be very tricky. The only real way to know is to talk about it. There is a difference between dating and being in a relationship with someone. When you really like someone you want to make it official and the more you keep seeing that person, the more you wonder "are we dating?" "Where is this going?"
It's a good sign when a guy keeps saying "We." It means that he is thinking about the both of you being together. "We" means relationship. A guy who is not serious about a girl will not use plural term.
When a guy tells you he wants you to meet his family and friends that is another good sign. Any guy wanting you to meet his family and friends is serious about you. He wants their approval and wants you to be a part of his life in a deeper way.
The only way to be 100% sure weather or not you are in a relationship is to talk about it. Don't jump into it and ask him right away because you might ruin it. Some guys are scared of commitment. If it has been going on over a month and you want to know where you stand, causally ask "where are we?" There is no time limit on to how many dates until you are in a relationship. It is when both of you feel comfortable to commit to each other.