[katy]

I've heard of using the "chalk test", but have never tried it:
Copied this off the web:

The chalk test is a method to visually assess if your tires are properly inflated by observing how a chalk line wears off the tread after driving. If the chalk wears evenly across the tire, it indicates proper inflation. If the center wears off more quickly, the tire is overinflated. If the edges wear off first, the tire is underinflated. Exploring Overland
Here's a more detailed explanation:
1. Apply Chalk:
Draw a wide line of chalk across the tire's tread, ensuring it covers the entire tread width.
2. Drive a Short Distance:
Drive the vehicle forward a short distance (e.g., 100-200 feet) on a paved surface, preferably in a straight line.
3. Inspect the Wear Pattern:
Observe how the chalk has worn off.
Even Wear: The entire chalk line should be visible on the ground, indicating proper tire pressure.
Center Wear: If only the center of the chalk line is worn, the tire is overinflated.
Edge Wear: If the edges of the chalk line are worn but the center isn't, the tire is underinflated.
4. Adjust and Repeat:
If the tire is over or underinflated, adjust the tire pressure accordingly and repeat the test until you achieve even wear.