The best way to locate the drain line connection is with a sewer camera equipped with a beacon and a locating tool that can locate the beacon.
If the drain lines are bad enough that you are considering replacing them, I wouldn't try to reuse any of them.
Just happened to me on a remodel. The GC wanted to leave as much of the old cast iron drainage as possible, so I did not run new piping to the kitchen sink, and I left the old vent piping in place. Under test, the cast iron vent piping had a leak inside of a poured concrete wall, so I had to abandon it and run a new vent line at 9 PM the day before the inspection.
