needing replaced, what would the symptoms be and what exactly does the switch do?

Okay, I assume you're talking about a sending unit for an oil-pressure gauge, and not a pressure _switch_ that just turns on a red light if the oil pressure gets too low (usually about 4 psi or lower).

The sending unit is not really a switch; it'* a rheostat, whose resistance changes gradually, depending on the oil pressure acting on a diaphragm within the sending unit (hence its rather large size), and its varying resistance is what gives you the different readings on your dashboard gauge.

If it springs a leak internally, its readings will go down, and it will often get soaked with oil on the outside. So if yours is wet with oil and there'* nothing nearby that'* dripping on it, replace it.

If you're trying to figure out whether a low oil pressure reading is due to a bad sending unit or Something Worse, like a failing oil pump or bad bearings, the very first thing to do is swap in a mechanical oil pressure gauge temporarily, in place of the sending unit, to see if it gives you the same readings. If it doesn't (i.e. if it gives normal readings instead of low ones), replace the sending unit and you're all set. If you do get the same bad readings with a second (presumably working) gauge, then the problem is elsewhere, such as a bad pump, etc.

In actual fact the symptoms and diagnosis are pretty much the same for a bad warning-light switch as they are for a bad sending unit, but I assume your car has a gauge in it rather than just an OIL light.