Welcome to the site, it is very good to see someone like you who takes a keen interest and trys to understand how everything works and what it does.
I dont have much experience with these alarms and, if they are capacitance alarms the normal way to test them is by immersing them in liquid, as previously said.
However, the capacitance level sensors on oily water seperators work through a Wheatstone Bridge type electrical circuit and there is a potentiometer to adjust the set point. When you take the covers off you may be able to simulate an alarm by adjusting a potentiometer inside, however, before changing any settings mark the original position, so you can return the settings to original. This will however only prove that the electronic circuit works, not that the probes do! If they are encrusted in dirt, there may be no change in the capacitance they measure in air and when in oil (both are insulators anyway).
If there is an adjustment inside the level sensor unit, the best way to verify that the unit is working is probably to remove it, clean the electrodes and place it in the liquid it should be measuring the level of, and adjust the potentiometer until it alarms, then check that it does not alarm in air. Make sure the tank wont overflow out of the alarm port when you remove it!!
It is always better to ask a stupid question than to do a stupid thing.