On creation of a sprite give its private variable an unique ID, I personally use the spawn object UID for that.
Then do this:
For each object <your sprite>
__If sprite private variable is equal <UID you need> -> Do stuff to sprite
[generally what Mr Wolf wrote]
I don't really think they'd need to use UID. How would they know which UID they'd need anyway? I don't see how this helps picking... Also, if they are just setting the object's Private Variable to the object's UID, then that is redundant. The object's UID already stores the UID...
Now if they were pairing two objects created in the same event, they could set each one's PV to be the other one's UID (or just do that for one of them depending on what you're doing) and that would work for certain things, but in this case, no.