If a non-Jew is tanning his own hides in the same area as a Jew who is tanning for Sta”m, the kosher hides must be marked so that they can be identified later. Preferably, the markings should not be forgeable, so that the non-Jew does not try to present his own hides as acceptable. B’dieved, even if the hides were marked with a simple symbol, there is room to be lenient. It can be assumed that the non-Jew will refrain from forging out of concern that the deception will be recognized.
[שו"ע לב, י, משנ"ב לא, וביה"ל ד"ה כעין וד"ה לזייף]