Never tried this before, but it strikes me as reasonable:
Paint a line across each tire. The paint should wear evenly across the tire indicating even tread wear.
If the center wears first, too much pressure. If the outside wears first, not enough.
For off-road use, you want a slight bulge.
Different loads (weights) will cause the pressures to achieve these pressures to vary.
Current: 2014 15 Passenger V8
Former: 2009 SMB 4x4 6.0