If you’re in the early stages of dating someone, you should always assume that the person you’re dating is still seeing other people as well, Firstein says.
But if you’ve gone on three or four dates, and you’re pretty sure you both like each other, just be honest about your feelings. (Yes, it can be intimidating. But you can do it!) Firstein recommends saying something like, “Look, I really like you and I want to know what you’re looking for.” And if the other person responds saying they like you, too, then you can reply with something like, “I’d like to keep getting to know you, but exclusively, if you want that too” (if that’s what you want).
However, keep in mind, if you've been on what feels like a bunch of dates with someone, and they're still seeing other people, Firstein says that could mean that they aren't ready for something serious with you. Again, you won’t really know until you talk about it.