I would recommend opening the pipe end and test for flow with clean water. Get a flexible funnel (from an auto parts store or wherever) and pour some water in to see if it flows away and doesn't back up. Or use a hose such as a spare washer intake hose connected to the washer feed. Note that if there is a trap in the wall it is likely dry and sewer gas may be present so some ventilation is desirable.

It's pretty likely the rough plumbing inspection wouldn't have passed if it wasn't hooked up. Your building department will have the info on that if you call or stop in.

thanks everyone. Once i unscrewed the cap, i was able to see that it is indeed the water drain line. It comes from the attic (maybe for the AC condensation, or the pan under the water heater, i haven't checked). The pipe is dry right now (no AC running since it's winter).

It could be vented to the roof.

If you hang the washer's drain pipe there, you might want to use some kind of strap to hold it in place.