Spend a few hundred bucks and hire a privet inspector for the area of the construction you are concerned about. if your not familiar with building codes meeting with the contractor wouldn't help you as fare as specific requirements in upgrade that affect the systems.
Experience is Knowing what to do next, Skill is knowing how to do it
When we rough in slabs, the only drain pipes you will see sticking up are 2" pipes and 3" pipes if there's a toilet upstairs.
We don't run 1-1/2" drainage in a slab, only vents. This usually makes it easier if something needs to be added to a line somewhere along the way.
that said, if there's a double vanity, you can get away with one drain pipe, and one set of waters. (waters can be tee'd off in the wall if needed, since realistically there isn't much of a problem running two side by side lavatories on a 1/2" line.)