First, learn what all the common defects are, e.g., diacetyl = butter, acetaldehyde = green apple, DMS = corn or celery or cabbage, oxidation = wet cardboard, chlorophenol = band-aid, other phenols = pepper, smoke, electrical fire, etc. Then, every time you taste a beer, search for these flavors. Sometimes they are obvious, sometimes you can detect something is a little off but can't nail down what it is until you think about it. Very often, there are no flaws at all, so be careful not to get too carried away -- sometimes a really good beer really is just a really good beer! Also, you might want to practice writing out BJCP scoring sheets to train yourself how to describe all of the flavors you are tasting. When I first got into BJCP classes and early days of being a judge, I wanted to score every beer I tasted! After doing this like 100 times, the nostalgia or mystique or whatever you want to call it finally wore off, so that now I kinda sorta judge every beer I taste all the time, but I don't bother putting numbers to it. With practice, the flaws (if any) will jump right out, and the really good beers will stand out as really good beers, AND you'll be able to describe exactly what you like about them. As with anything, practice makes perfect. I would also say that taking a BJCP judging class was one of the most eye-opening experiences of my life. It's awesome. If you're thinking about it and the opportunity comes up to attend a class, jump on it. You'll love it. But I do think it is possible to be self-taught. Probably will just take more time and self-determination. Learn the faults, and memorize a flavor wheel, and practice diligently 100 times, and you'll get the hang of it.