How to test a pond filter UV lamp and ballast?
That is a germicidal (UVB) UV lamp. I have a similar one in a UV EPROM eraser. 

Here is what it looks like in a fairly dimly lit room: 



It might be hard to see outside on a bright day, but it should be easy otherwise. 

I am defeating the safety interlock with a pair of tweezers. There are risks in this as UVB can easily damage your eyesight. 

If you are measuring 240V at the lamp socket with no lamp installed, the ballast would appear to be okay. This type of bi-pin lamp contains a capacitor that causes the filaments to heat at turn-on thus acting as a starter. If you are still concerned about the ballast you can test it with a similar but visible light 9W CFL lamp.