You don't want to simply test the tubes. Tubes perform differently at different operating voltages. What might pass as acceptable at one voltage might fail at a higher one. Just about all tube testers operate at a voltage that is far below what your 400+ operates at. That's why the amp itself is the best tester as B-string said.
You can send them to Eurotubes for testing. Or send them here
, they offer a tube testing service. They use the same tester as Eurotubes. More info on their site as to how they test the tubes.