This last weekend I had the opportunity to participate in an off flavour training seminar organized by the Texas Craft Brewers Guild. Sponsored by Real Ale and hosted at Independence Brewing, the class was designed to help identify common flavours and aromas in beer that are typically undesirable (although, depending on the style, some are not only desired but even required).
The goal is to help train your palate to identify these often subtle flavours and aromas, so they were presented to us in an exaggerated and sometimes overwhelming state. Tim Shwartz of Real Ale lead the seminar and walked us through each sample and discussed what causes the different flavour or aroma, where it might be appropriate and steps to take for avoidance. Samples of a common and cheap macro lager were spiked in order to experience the various off-flavour profiles and small vials containing various noxious potions were handed around to sniff to experience some of the aromas.
A word of caution though, once you take a class like this, it may change your opinion on beers that you've enjoyed in the past as you'll begin to notice these off flavours a lot more. There were a few "a-ha" moments during the class as people identified flavours in beers that they consumed recently but now recognized as one or more of these off-flavours. Not everyone can detect all off flavours and aromas however, so don't be surprised if someone raves about a beer that you clearly detect butterscotch (Diacetyl) or they notice a corn or vegetable aroma (Dimethyl Sulfide) and you don't. If you're going to the GABF, check around the middle of the convention floor near the Brewers Guilds as they often have a booth setup to try your hand at identifying off flavours.
If you want more information regarding off flavours and aromas, check out this PDF file available on the Beer Judge Certification Program (BJCP) website. If you're interested in attending a similar class, check with a local homebrew club or homebrew supply store and see if they offer any or if they'd be interested in organizing one.