How do u locate down drain pipe in bathroom with dual sinks behind walls

Hi Billy and welcome to the Plumbing pageat at AskMeHelpDesk.com. You piggybacked on an a 4 year old dead thread so I gave you one of your own. In the future, before you post, look in the upper left hand corner for the date.
Exactly what are you asking? Are you asking what's in the wall? The drain and ventfor both vanities should be found directly behind the trap. If it's not there it will be within 5' of the center on either side. Why do you wish to know. Regards, Tom