This is a very good question because when you are inflating tires, you need to be careful that you do not overinflate them. Doing so can cause more damage than good but you also want to ensure that they are at an optimal PSI level. Also remember, though, if the tires are not pneumatic then you will never need to worry about their air. But, if they are, there is one sure way to know the optimal tire pressure that is needed.
The recommended PSI should be specified by the manufacturer either on the warning label on the rim or in an included manual (or online on the product page). This is important because when the temperature changes, tire pressure can change. Your tires will not always be inflated to their ideal amount so keep an eye on them just as you would a vehicle.