How do you know if it's time to replace a UV lamp?

[Delphinus]

I swear the eerie little glow that comes out of my AquaUV is dimmer all of a sudden (I had to unplug it a couple days ago to get at a fish who had sumped himself). It's a unit with a wiper so the quartz sleeve is clean, so that's not it.

It's not like you can take the lamp out and check out the brightness that way, since UV is bad and all, so I guess my question is, how do you know if it's time to replace a UV lamp? Do they get dim? Or is that a figment of my paranoid imagination? Can I hook up the unit to a watt meter and if the watts used are less than the rating maybe that could confirm it's dim?

Or do they just burn out and that's when you replace them?

I have about 10 months of use on the unit.