Well, there is a whole beer judging program where people spend lots and lot of time learning to taste subtle flavors - both positive and negative. It really doesn't come naturally to most people.
If you really want to learn them, you should try an exaggerated version of the flaw. Once you taste the extreme version a couple times, your taste buds will zoom in on the subtle version MUCH easier. It's kinda like trying to tell someone what bubblegum tastes like. It's pretty challenging to describe, but once you've tasted it, you'll know that flavor anywhere.
Of course, most people don't want to purposely brew bad beer just to learn about flaws. You might want to search around your local brewstore and see if anyone does classes on the topic. One of the ones in town does a "Tasting off flavors" class once a year or so.