You ask around. Ask you friend, "Hey what are you up to tonight?" or "Hey, are you going out tonight?"
If you're a freshman, it's going to be a lot harder since parties are often thrown by people with apartments, aka sophomores and upperclassman. The vast majority of parties I went to as a freshman through people who knew older people. Often times I would only know my roommate and maybe one other person out of the whole party (they were still plenty fun tho).
I know that's not very helpful, but that's really the best/only way to hear about parties if you don't personally know anyone with an apartment. To get in the loop you have to put yourself there, so to speak.