Assuming you don’t want to just leave it to chance and hope that everyone is on the same page, you have a conversation about your relationship during which you explicitly talk about exclusivity. It doesn’t have to be some big, heavy thing like, “hey, we need to talk about our relationship.” It’s as easy as saying something like, “I’m really enjoying our time together, and I want you to know that this is the only relationship I’m focusing on right now. How about you?” Unfortunately, a lot of people just sort of assume that its obvious when a relationship is exclusive (or they assume that all relationships are automatically exclusive). This is why it’s important to have an explicit conversation, but you also risk upsetting someone by treating it as a question at all. I’m not particularly interested in people who can’t have that kind of conversation, but there are ways of backing into the discussion if you’re worried about how your partner might react to the direct approach. One way is to just say something like, “I was telling one of my friends about you, and he was jealous that I managed to find such a great girlfriend.” The key point is to drop the word “girlfriend” (or “partner” or whatever you both prefer) into the conversation in order to see how she reacts to it. Is she happy that you called her that? Does she reject that way of characterizing your relationship? Either way, it opens up an avenue to further discussion. But honestly, the direct approach (which does not have to be blunt) is probably the best.