The inside of the headcap is stamped and will have a sharp, clear image like this:
This card is called a "Guarantee Card" and has the name of the sculpt with it's resin color written on it, as well as a serial number. I haven't checked it out myself, but I'm assuming the serial numbers are unique to the specific head you have, rather than the entirety of the sculpt. I've blocked out most of the serial number on mine so it can't be copied easily.
Thanks for reading! I hope this was useful. :) Be sure to ckeck in next time for a post on Identifying Recasts LLT.