Go to a flat surface and take some chalk. Draw a line across the tire and drive 100 feet or so and back. Get out and look at the chalk line. If it's worn off on the inside of the tire but not the outside that means you need to air down, vice versa. You keep doing this until it wears evenly.
Now the chalk test doesn't meant it's the PSI to give you the best ride.. It's just the PSI to give you the evenest wear across the whole tire. So it could feel to soft for your liking and you may want to sacrifice comfort over wear. It's really up to you.
It's also very time consuming and trial and error. And this will only work if the tires and wheels are even in size. If the tires are wider and on a skinnier wheel the test really will not give accurate results.
In all honesty it's all going to be different for each person and each vehicle for each tire/wheel combination...
I was wrong when I said to always use the chalk test... instead, always use common sense and what feels the best to you. Obviously if the chalk test says to run at 16psi that's not a smart move(on road)