If theres 2 facing logic says one is a photodiode and the other is a infra red led.
Use a mulitimeter on diode test and check them both, the one that registers around 2v is the led, you can then if you like connect the led to a 9v battery via a 1k resistor and 'look' at the led with your camera phone or whatever, you should be able to see it lit up. You can then 'shine' the led on the phototrany (or photodiode) while measuring its resistance with said meter, it should drop the more its illuminated.