How can I tell if a given roof vent is still active, connected?
One thing you can do regarding the question if it an active part of your plumbing is to turn on your water sources one by one. While water is running down the respective drain put your ear to this unknown vent pipe top and listen for the sound of running water.

One way to make this process efficient is to have two people talking on cell phones. One person is on the roof to listen at the pipe. The other person operates each sink,  toilet or washer in turn while communicating with the person on the roof.

Of course use common sense safety strategy climbing to the roof, especially if it is particularly steep of high from ground level.