This is especially problematic for beers in clear or green bottles because they filter out very little if any light. Unprotected beer can be “skunked” in a matter of seconds and can easily happen to the freshly poured beer in your glass especially if you are outside enjoying a beautiful sunny day.
Unfortunately, there is no way to tell if a beer is skunked before you buy it. You can reduce the chances of purchasing a skunky beer if you avoid clear or green bottled beer that is sitting out unprotected in the sunlight or under florescent lighting. Reach for the bottles packaged in boxes or the ones toward the back of the shelf in the dark. If you do buy a beer that has been skunked (assuming you took good care of it) return it to the store and ask for a replacement.