laundry renovation washing machine standpipe redo?
I have a 1960s home with cast iron drains and the laundry room on slab. The current standpipe to the washing machine box has no (visible) trap or vent in the wall, for at least 4 feet on either side. The pipe itself is either 2" or 2.5" and goes straight down into the slab. It MAY be tied into a lavatory drain vent in an adjacent half-bath (but that's at least 4' away).
I know adding a vent wouldn't hurt, but I want to do it on the proper side of a trap, and I don't know if I even HAVE a trap. How can I identify/locate a trap in a slab, or is that most easily done with a plumbing video scope?