If you don't have a 6BL8 handy, you can try using the one used in the delayed sweep circuit (V261, the Miller integrator circuit is identical)

Remember that tunnel diodes cannot be tested as a normal diode with some diode tester. A curve tracer (and GREAT precautions) is mandatory.
Apart from that an excessive current from the digital multimeter could burn it, they would result as a short in both directions, so it's impossible to determine they are good or not.

To check if they work just see with an oscilloscope if they switch (approx 200/300 mV between OFF state and ON state).
Do not unsolder them unless you are reasonably sure they are faulty. They are very delicate and almost impossible to find.

Max