I think you can just unplug it and, if the radiator light goes off, 99% you found the problem. In my experience, it's not so much the sensor in the bottle that goes bad as much as it's typically that the float stops floating, and is always sending a signal that the tank is low/empty. I've been able to visually inspect the float in the tank and verify the problem that way as well.

EDIT: I'll throw in my own "21 month" update as well. The grommet is still working perfectly and there are no leaks from my coolant tank. IMO this is a cheap and easy enough mod that it should basically become preventative maintenance when getting a new to you van if the seal looks at all worn or you don't know history of the van.