They usually consist of an IR LED and sensor, although occasionally you'll run into ones with visible LEDs. A blast of compressed air from a lens cleaner spray can will often do the job if it's just a matter of dust.

If you're able to apply power to that thing, you can use a voltmeter and measure whether the sensor is actually working. One of the pair will have an output voltage that depends on the presence/absence of paper in the slot. The other pair (that corresponds to the LED's connections), will just be a constant DC voltage.