Alcohol is produced by various yeasts metabolizing sugars in fruit and producing ethyl alcohol as a by-product. The sugar is found one of ways:

• The juice from fruits
• Grains germinating
• Grain starch being converted to sugars by mold
• Root starch being converted to sugars by people chewing on them and spitting them out

All of these were food sources even before their domestication, so in theory anyone of them could have been the source of an accidental discovery of alcohol. Its possible that alcohol was discovered by accident in pre-Neolithic societies but it is interesting to note that in the Americas, in places where you had widespread agriculture such as Mexico and the southwest of north America you have cultures that produced alcohol. But in areas where lifestyles were more classically "hunter/gatherer" there is no evidence of alcohol production. So its possible that other areas followed the same pattern.

Agriculture has its first known genesis in the "fertile crescent" region so its possible that this were alcohol was first discovered. The oldest known archeological evidence of alcohol is in China however.