I bought my house with a pool and have no clue how they ran the returns, other than seeing where they come out and guessing from that. I have one separate 1.5" line that was a cleaner motor, so I know which return it is. I do not use it and was planning on using it to hook a solar panel into (externally, underground). I started digging outside the pool right above the return I want to tie into, came to a 1" line (about 12" down) that goes to a further return. The return in the pool is 14" from the coping. I am at 18" and see no signs of it going into the pool. I am up against the concrete wall on the outside. They wouldn't run a return inside the concrete would they?
My next step is to turn off the pump and remove the return nozzle and see if I can tell by running a stick into it... I have my Poolskim on the further return, so don't want to tie into that one. Not sure if the 1" pipe is large enough anyway.
edit: strange, stuck a stick straight into the return and seems to stop right at the outside surface of the concrete (within 1/4"). Sticking a flexible hose in it seems to take a 90 at that wall and head down. All this must me happening inside the poured concrete... guess I'll forget tying into it... in theory I could tie into the return that the Poolskim is on on (T-off to solar and back into that return). That way Poolskim will still work. Not sure if I can get 4gpm to solar (at about 8' up) through a 1" line.