The easiest way is to pour water into the sump pump pit, a sufficient amount to activate the pump switch. If you have a large sump pump pit, garden hose might be the fastest way to fill it up. This will test the pump and its activation switch – the cleanest and the most “real life” resembling test.
Second option is to remove the plug that goes first into the electrical socket and insert only the second one, the pump should start as soon as you do that – this will only test the sump pump motor, not the switch.
Another way of testing the sump pump and its switch is to pull out both plugs from the electrical socket (don’t separate them), clean the little plastic tubing and surrounding area on the switch cord plug (it should be the one that was closer to the electrical socket), suck out the air from the tubing to activate the switch – once or twice is usually enough (you might be able to hear a quiet thump when the switch activates).
Quickly cover the tubing with your finger and insert both plugs into the electrical outlet the same way as it was before you pulled it out. The sump pump should start pumping and you can remove your finger from the tubing. If for some reason this last test fails, try it one more time, maybe the switch didn’t hold.
Let me know if you have any other questions.