what kind of tires are they??
Are they R1 ag tires, R4 industrials, or are they trurfs??
Most of the ag tires run about 8-12psi in the rear and 25-30 in the front. The R4's I think run closer to 18-20 in the rear and about the same in the front.
It should have a max pressure marked somewhere on the sidewall of the tire. Just so long as you have enough air as to not bulge the tire of roll it off the rim without going over the max you will be okay.