I would let the engineer who designed the slab do the "real" inspection. I have, however, inspected the following before pour and sometimes before base: Measure the drain placement (have found them 8' off - plumber forgot patio was there), check for conduit, drain, water supply line to islands, measue toilet drain distance to wall, check for downdraft fan duct, possible wate heater drain and TPR drain lines (depends on where water heater is located), etc. I have saved plumbers alot of money from measurement mistakes! The items I mentioned should be checked by the builder!