Does anyone have a trick for telling which rough-in stub is the loop vent and which is the waste line? I found plumbing under my kitchen island and would like to add a bar sink. Typically a loop vent connects back to the waste line after the vent to roof lateral.

It was at one time the way a combination waste and vent was done. A 2" simply stubbed up twice and connected at the top. Either one can be used. They go to the same place.