If the drive shaft is worn, you can see the small groove where the seal rides. When I pressure test, I use a foot bicycle pump, it has a gauge on it. I pump it up to 12 psi and leave it for an hour. After an hour has passed and if the gauge is still at 12 psi, I figure it's good. (you can also check it for leaks with soapy water in a spray bottle.) Then I use a vacuum brake tester and pull vacuum on the gear box. You need to "push, pull" the seals when checking them.