How do I test the functionality of a sump pump?
There are typically "sliders" or some sort of adjustment that mark the "start" and "stop" (or "high" and "low") water levels.

Generally there's no minimum low-water level other than you want to make sure that there is always water covering the pump intake (so it doesn't suck air).

The "high" mark just needs to be low enough that water never gets out of the sump.  If you have a "high water" alarm (and you should), make sure the "high" mark is low enough that it doesn't trigger by accident.

Other than that, the further apart they are spaced, the less frequently it will turn on and the less wear on the pump (start-up is the most stressful time).  Too close together and any backwash down the output pipe could cause it to turn back on again in an endless cycle.

For testing, dumping buckets of water down is fine, as is using a hose to fill it.  You just want to make sure that it turns on and off within the limits described above.