Yep, the volume of air is reduced, but I would keep the same pressure. Regular tire gauges work, but getting any liquid in them may cause premature corrosion, so they make gauges specifically for air/liquid and they cost little, if any, more than any other gauge.
My personal preferred method to check the tire air pressure with loaded tires is to (1) stop with the valve stem at the top; 12 o'clock position, (2) give it one short blast of air from the compressor to clear the valve stem of any moisture, and (3) then use the tire gauge.