By the process of elimination. You can hot up all the circuits and then the one left will have 2 dead ends (unless you fed it in the middle).
If you used plastic nail-on boxes, you can carefully pull the nails on them by inserting a screwdriver at top and then bottom and a little at a time(and as you pull the edge of the box out opposite the nail side) get it free.
Then you can (unless you have real large hands) search for the cable. The nail-on portions of the box can then be snipped off with dikes and the box remounted with drywall screws thru the side.
A mid-line or even jumbo plate may be needed to cover the edges, or a bit of patching. I have even changed to a 2-gang and double duplex to avoid patching.
I have had to fix all sorts of helper/careless stuff. You'd be amazed what can be done sometimes.
There's no substite for a thorough going over before they "make it white".
