Air vent pipe or water pipe?

I have someone who wants me to frame in their basement. There is the rough in for a sink and toilet. The person wants me to remove and cap both so that flooring can be laid down eventually. Actually, the toilet one is already capped and my question is about the black plastic pipe for the sink. Sorry I don't have a good pic.

What I am trying to determine is if the smaller balck pipe that comes out of the floor and then disappears into the ceiling (there is a capped pipe coming out the side of this one which I assume is the drain for the sink) is simply for air venting or if it is in use for another drain somewhere else in the house. I can't hear anything (ie water) when other sinks are turned on.

How can I determine the function of the pipe?