I've seen lots of threads where people have said that a single-wide breaker means your pump is 120V and a double-wide breaker means your pump is 240v. That's not always true... Who here hasn't bought a house and run into a few homeowner special modifications?
I replaced the booster pump on my Polaris earlier this spring. The main pump has a double-wide breaker in the middle of the 6-slot subpanel. A single-wide breaker in slot 2 is labeled "Polaris.". So I figure the booster pump is wired for 120V, and I start the job of replacing the old pump. Fortunately I'd taken the step of wiring the new pump for 120V before working on the old pump, and knew something was up as soon as I took off the cover plate and saw the old pump wired for 240V. I start poking around with my multimeter (smart) but manage to short out the probe and get an explosion of sparks in my lap (clumsy). Startled but unhurt I go back to the panel and find breaker 6, labeled "filter," has tripped. At least I now know how the pump is wired. Keep in mind that I've only had this house and pool for 2.5 years, so I am still finding surprises. The attic exhaust fan was another fun one, hah.
So the moral of the story is take nothing for granted, and be safe. Verify that power is really off when you're doing work. Some electrical work can be quite easy, but don't do it without the proper tools and precautions. A cheap $20 Sears multimeter can be indispensable and should be in every homeowner's toolbox.