since wheel width, load weight, and other factors all come into play the best way is time consuming but it is the most accurate way is to draw a line of chalk across the width of the tire about 2 inches wide. set your tire pressure at what you think is about right. Then drive it down the road and back. Check for rubbed off chalk. This exposes what part of your tire contacts the ground the most. Worn off on the center= too much pressure, worn on the outsides= too little pressure. Ideally you want all of the chalk to be gone.

-Chad