There are some tests that you can run. If you have some machinery around, for example, you can pressurize both sides of a cylinder to test the effectiveness of a piston seal. Or you can, say for example, pressurize one side of the cylinder with no hose attached to the other port and the rate at which you see fluid leaking across that piston and pouring out that other port gives you an idea of how effective that cylinder seal is. You can change seals out. Literally just watch for leakage and try different applications.
The other thing you can do is install it … if you have a pilot-operated check valve, you can install a cylinder up, put a load on it, and see how long it takes to drift down.