Basically you put a few lines of chalk across the tread and drive it a bit to see how evenly the chalk has worn. If the chalk is more worn away on the edges, the tire is underinflated. If the chalk is more worn away in the center, it's overinflated. However, the chalk test isn't as foolproof or all it's made out to be. It can fool some people into underinflating their tires below what they should be.
Like in my opinion, 25 psi is too low of a pressure for radial 30x9.50x15 tires (bias belted tires do well at lower pressures). Heck I run higher than that, 26 psi, in my 35x12.50x15 tires which over two sets of tires has produced absolutely even wear. The factory's recommendation for a 30x9.50 tire on an unloaded Wrangler is more like 31 psi and 33 psi when the Wrangler is fully loaded with people & luggage. That psi is based on the tire's size, load carrying rating, and the weight of the Wrangler. The chalk test alone will often mislead people into underinflating their tires so while it can help (especially where overinflation exists), it shouldn't be the sole indication of what pressure to run.