We must deal with a few issues.
Some of the main kashrut concerns regarding agricultural produce pertain mainly in Israel – notably, land-based mitzvot (e.g., teruma and ma’aser, Shemitta, orla) – and are of little concern outside of Israel. But in the case of wine, stam yeinam is a major remaining issue. Stam yeinam is a Rabbinic prohibition on wine that was handled by non-Jews,[1] starting from the time the grapes are squeezed and even after it is produced (unless and until it has been mevushal – literally, cooked[2]). Unsupervised wine produced by non-Jews is therefore certainly forbidden as stam yeinam, whose complex nature we will now briefly highlight.
One part of the Rabbinic prohibition of stam yeinam is the concern that drinking such wine could lead down the line to social interactions and intermarriage.[3] (This concern is also reflected in similar prohibitions, such as bishul akum.[4]) If, however, the wine was actually involved in idol worship, it could become yayin nesech, from which it is forbidden to derive any type of benefit. The latter is forbidden on the level of Torah law.[5] Because these two prohibitions can be confused with each other, the Rabbis added a prohibition on receiving benefit from stam yeinam as well.[6]
There are sources that indicate leniency regarding the Rabbinic prohibition on receiving benefit from stam yeinam. The Shulchan Aruch[7] rules that wine of a non-Jew who is not involved in idol worship is forbidden only for drinking, not for receiving benefit, but it is not clear what category members of various religions fall under. The Rama[8] writes that since it is not common for non-Jews to use wine for libations, not all agree that there is a prohibition on receiving benefit, and one can receive benefit if needed to avoid a monetary loss (e.g., when the wine is the main available asset of a non-Jewish debtor).[9]
Wine tasting, even if one spits the wine out, provides benefit due to the taste, and it is therefore forbidden if there is no potential loss. Not taking part in wine tasting is not a loss of money, and the loss of a pleasant opportunity is not considered a loss in this context. (Even being precluded from doing commerce in non-kosher wines is simply a lost opportunity, and it is thus forbidden.[10])
If stam yeinam were only forbidden to be drunk, as in a case of monetary loss, we would have to analyze the halacha of putting forbidden food in one’s mouth and then spitting it out. (It is considered cultured to spit out wine into a spittoon at wine-tasting events.) According to the great majority of sources, it is forbidden to taste foods that one is forbidden to eat.[11] One of the main sources for this concept is the gemara’s halachic advice for someone who is unsure whether a particular mixture of kosher and non-kosher foods is permissible. Since discernable taste of the non-kosher minority element would make it forbidden, the gemara[12] says that one should give it to a non-Jew to taste. This implies that a Jew may not taste the mixture, even if he plans to spit it out. On the other hand, Rishonim[13] allow a Jew to taste a piece of liver to see whether the animal has a hidden gallbladder (important for the laws of tereifot), even though the liver would be forbidden if one were to discover that the gallbladder is missing.
Several possible distinctions are raised to reconcile these sources, including the following: 1) One may taste the liver with his tongue to see if it is kosher, but he may not taste it in his mouth.[14] 2) Since the liver in such a case is almost always kosher, it is permitted to taste it to make sure it is.[15] However, the consensus is that it is forbidden to taste a fully edible prohibited food by putting it into the mouth, even if it is forbidden only Rabbinically.[16] This is even clearer if one is sampling the food in order to enjoy the taste.
Therefore, for two reasons, you may not taste the non-kosher wine.