How should my players realize they misidentified an item?

One of my players misidentified a Pendant of the Occult. I told him it gives the +1 to Occultism checks, but also told him it can be used on a target to give a -1 status penalty to their next attack roll, Perception check, saving throw, or skill check (basically reverse-Guidance).

I immediately realized I didn’t make the effect obvious enough, so the players probably won’t notice that the Pendant is having the opposite effect unless I add some sort of descriptive flourish to make the effect blatant.

Is that what I should do, or is there any rules guidance about how (or when) PCs should realize their misidentification?