[RodB]

There are circumstances where you would be able to test a cap in circuit. You need to have one end out of the circuit otherwise other components will interact and make the measurement unreliable. If it is connected as the only component to a tube element then you could run a test on it. If the cap has a high DC voltage on one end it should block the DC from passing through, so you can measure the DC voltage on both sides to check if it is leaking.