Ad for a tube tester. These were found in drug stores, grocery stores, and other venues. You would rock up with some tubes, and plug them into the right socket on top. Then the tester would tell you if the tube was good or bad. If bad, it had drawers full of new ones, which you could purchase to replace your bad tube.

When transistors took over, there was no longer a need for tube testers, and they faded away by the beginning of the 1970s.

Here is an RCA branded one in the flesh.