I've got several different size returns in the pool. At one point, the pool was drained and the fittings were removed and never replaced. All I've got now are pvc pipes of varying sizes that lead into the pool and end at the concrete. The largest opening is threaded, but no water comes from there. One is tiny, maybe 1/4", almost like a weephole, but also has no water flow. The rest (4), all unthreaded, are pretty close in size, with some variances. I've only measured it by sticking my finger in the return and comparing the "fit". My finger blocks some completely, while others leave gaps. None seem to be anywhere close to 1.5" The return furthest from the pump has a weak flow, while the one closest to the pump has significantly more flow, but of course the water is directed straight into the pool, and not toward the skimmer or drain. I suppose it's possible that the piping I'm seeing is the remnant of an older fitting, cut off at the pool's walls, but I can't tell that there is any diameter change within two inches or so from the pool's wall.
I'd like to re-install some eyeball fittings, and I'm thinking of using smaller eyeballs closer to the pump so that more water pressure is directed to the far side. Any suggestions on measuring the pipe, other than strapping on a mask and using a ruler?