Originally Posted by papakevin
. . .leading me to believe they tapped directly into the septic and didn't install a p-trap.
Is there an easy way to check to see if one is installed other than cutting into the drywall?
If there is a p trap that hasn't dried out and you force air into the pipe you will see a back pressure of one or two inches of "water column".
So, the trick is to make a transparent flexible p trap and fasten it to the output of the pipe with an airtight fitting.
Then, if you raise one end of your homemade manometer and the water level tracks the change then there is no trap downstream or the trap is dry. Dump some water in and try again.
If the water level resists readjusting its level then there is a trap or blockage downstream pushing back.