How to 'label' module using electric techniques
If you are unable / unwilling to change your interface into a purely digital system whereby a small microcontroller in the sensor module interfaces the sensor with a communication bus, the best method is probably to include a small 1-wire EEPROM with each module.

An ideal one would be the DS2431 which provides 1024 bits (128 bytes) of EEPROM storage using just 1 wire to both communicate and provide power.

Programming in a unique identifier for the module would be a simple enough task, and that unique ID can then be used to work out what the module is.

The DS2431 is pretty cheap too, available at prices around  $1.50 and below (much less for bulk and surface mount).

You can also get bigger versions for storing more data should you want to.