Does the horseshoe do something easily verifiable during the epic quests or does it require number-crunching to verify?
E.g. if it made it so that you teleported from one quest NPC to another quest NPC at certain points of the epic story (rather than making you walk), this would be easily verifiable by running the quests with the horseshoe on one character and then without on another. However, if it gave you a +10% crit chance that doesn't show up on the character sheet, you'd need to analyze combat logs and do some statistical analysis.