We would like to put a bathroom in our finished basement where the current washer/ dryer are and relocate the washer and dryer to a nearby closet. We were told if there is going to be a toilet the drain line could be no less then a 3" line. While we know its not 3" in the wall we are not sure if it branches into one under the floor slab. We have reached out to the local government to see if they have the plans for the house... they do not.. Is there a way to determine the size of pipe under the floor without cutting the concrete to look?
Unless your original builder cut corners, the entrance and exit will be the same size as the pipe under the floor.
If they ran a smaller pipe for a section in the floor, that would require two adapters to mate up the smaller pipe with whatever you see. That would cost more than just running the larger size of pipe.
It is not yet a toilet, so don't feel squeamish about sticking your hand into the pipe and feeling as far down as you can. For a rough gauge, most adults would have problems fitting their knuckles into a 3" or 75mm pipe, or you can test your hand against a known pipe that you can access.
You can use a camera, but they're not perfect. Instead, get a lightweight cylindrical thing of about 2 4/5" (say 70mm) and secure it to a flexible rod or wire. You could even use a party balloon inflated to that size.
Then try and "rod" the pig through the pipe, feeling for blockers like a narrowing.
If you could get a draw-cord through the pipe, then pulling a pig through is easier. I wouldn't suggest a 3" or 75mm pig, there could be flanges, globs of sealant, etc which may give a false answer.
Please, do tell us how you get on - closure is important in Q&A.