Bristlebane's Soothing Wine  

The bread and wine from the jester each come in two forms. They look identical if inspected, but are actually different items. I don't know what the odds are of each, quite possibly 50/50. All doses in the stack of 10 will have the same effect, so you can try a dose and throw the rest away if it's bad. But that still leaves you poisoned if it turns out to be the bad kind.

Here are several ways to avoid being poisoned, while still getting to use the good bread/wine when you get it:

Easiest is to have someone cast a Cure Poison on you if it turns out to be poison. Of course, you might not have anyone who can do that.

Go to a combat-free zone such as PoK or PoT and try a dose; if you're told "tried to cast but you are protected", it was a detrimental effect . (This method is particularly handy when someone summons a jester at the PoK bank.)

Write your inventory to a file using "/outputfile inventory filename". The two versions have different ID numbers even though the names are the same. The odd-numbered versions are poison (they taste odd).

Check your Magelo inventory for even vs odd, or follow the links there to see the spell effect.