Troubleshooting a round fluorescent bulb with 4 pins in a square formation?
Sometimes there's two buttons: hit the ON button for a few seconds, and
watch the ends of the lamp (you may see a glow from the heaters).
If that doesn't start the lamp, there may be an automatic starter
(look for a small aluminum can, these are essentially thermal delay
switches that mimic the manual press-and-wait procedure).   The
starter will be bayonet-mounted, press/rotate and it should pop out.
Remove and reinstall it (checking that it isn't loose).

The lamp presumably has two filaments which heat to start the light,
and those will have some electrical conductivity - you can check them
with an ohmmeter (should read just a few ohms).  There's a gas-filled
envelope.  That, you can check with a Tesla coil, if you have one, or by
popping the tube into a microwave oven with a jar of water.   It'll
light up in seconds if it hasn't leaked.