Performing a leak down test can provide you with more info on cylinder sealing.
Here's a tool for removing valve spring on the head. When doing this You have maintain the valve up against the seat. You can do that with the leak down tester by pressuring up the cylinder when its at TDC. I've also seen people use some thin rope. They use a narrow funnel to drop the rope into the cylinder through the spark plug hole when it's at BDC, (but not all of it you have to get the rope out, leave some hanging out), then they turn the crank, to TDC. The rope basically fill the void and stops the valve from falling down.