Mouse IR detector Challenge

Anybody know how to determine which legs are which on the IR detector (double transistor) in old-school mice? One leg is common and the other two legs are emitters. Is there a test I can perform with a needle type VOM that could identify the leads? These three legged components are very common and can be used to build really simple and cheap data acquisition of rpm using the microphone jack and 5 volts from a USB port. One leg has extra metal and is likely the common. Still I would like to know if anyone has a test procedure. Thanks to all and keep repurposing everything in your path. It just feels good.