Washing mashine drain

I am adding a bathroom next to the laundry room and my contractor believes that the washing machine drain line will turn from a 2" line to a 3" or 4" line under the slab. How can I tell for sure. I would hate to cut the concrete only to find out that I can't use it.

No good way to tell. It eventually will get to at least 3". Where is it in relation to the other things in the house, and where is the main outlet where everything comes together and leaves the house? The pipes only get bigger, so as you move in that direction, eventually, it will be bigger. Hard to tell where though. My unprofessional opinion.

Not before the next tee.

The usual place for an increase in size is where another drain tees into the line. The size that goes into the floor will probably continue to the next tee, whether the new drain comes from above or under the floor.