Sidewall of the tire will have the max specs for the pressure. You need weigh as said then determine your payload needs, adjust according. Pressure rises as heat increases....check as you go down the road.
You do not need to check as you go down the road as the rise in pressure as they heat up is part of the tire design. The pressure in the tire is to be measured when "cold" which means before the sun heats up the tire and before you drive 1 mile.
On a hot summer day, I have seen my cold 80psi tires rise to about 110psi after 200 miles and the temps going from 65 to 105.
