I agree with @CalMeacham; it the xenomorphs are the first aliens humanity has encountered, that certainly wasn’t clear to me. Beyond the extended universe material @Larry_Borgia references (in which it’s very clear that the xenomorphs are far from the first aliens humanity has encountered), the internal evidence all seemed to me to point to previous encounters with aliens.
Go back to the original Alien. The crew of a random freighter is sent to investigate an anomaly that turns out to be an alien spacecraft. No one seems particularly surprised that an alien spacecraft exists. This seems like humanity’s first contact with that particular alien species, but it hardly seems like the first contact with any alien species.
And beyond the internal evidence CalMeachum points to, there’s the reaction of the Colonial Marines themselves. They’re surprised by the capabilities and intelligence of the xenomorphs, but not by their existence. “They’re just animals, man!” If this is humanity’s first encounter with an alien species, why would Hudson assume that they’re just dumb animals? That line really only makes sense to me if Colonial Marines had gone of plenty of “bug hunts” and fought animalistic aliens.
Or the Company debriefing with Ripley. The execs don’t seem to me to be incredulous that she reported encountering an alien, they seem incredulous about the details of the encounter.