If you can't find a datasheet for the module, you should be able to simply measure it with a voltmeter - see if the voltage on the pin is high or low. Then swipe a barcode, and see if the pin changes - if it was low, and then goes high, it's positive logic - and an LED should be connected down to ground (possibly with a series resistor - depending on the module). If it's high, and then goes low, the LED should go to the +ve supply (negative logic).
There's also a possibility that the output is open collector, in which case further tests would be required - but I would suspect not, as it's intended to signal the processor of a valid read.