To answer your immediate question, the best thing to do is to go through a well-established music database and listen to representative tracks from each genre: Amazon, the iTunes store, Pandora, Last.fm, Spotify if you’re in Europe, SoundCloud, take your pick. Figure out who the ten or twenty most popular and influential artists are in each genre and listen to as many of their songs as you can. Once you’ve listened to a wide variety of songs in a given genre, you’ll get a sense of their defining musical characteristics, and you’ll be able to make judgments of your own.
There’s a bigger question behind this one, though. What does a musical genre really mean? The various genres are vaguely defined at best. For example, the boundary between soul, funk, blues and R&B is a soft one. Let’s say you’re trying to name the genres for the tracks on a single Aretha Franklin album. You’d be hard pressed to place them into the soul, funk, R&B or blues categories unambiguously. Some of Aretha’s work could also be categorized as pop or gospel or jazz. So do you start hyphenating? Do you create big vague categories like “funk/soul/R&B” or “blues/gospel/soul”? Do you create a long list of more specific categories? Do you find a way to include a track in multiple different categories? The more you try to categorize music, the more taxonomic issues you run into.
It’s funny to go through the music services and see that there are dozens of ultraspecific microgenres of electronic music, and then you get to jazz, and there’s just… “jazz.” It’s silly to have a single descriptor for everything from Billie Holiday to Cecil Taylor to Weather Report. I break my iTunes library down into “jazz (vocal)”, “jazz (free/avant-garde)”, “jazz (fusion)” and so on, and even so, those categories are inadequate for edge cases. What do I do with the seventies Miles Davis albums that are both free/avant-garde and fusion? Should “jazz (fusion)” cover jazz-Latin fusion, jazz-bluegrass fusion, jazz-Indian-classical fusion, jazz-rap fusion? Or do I need separate genres for all of those too?
The same thing happens in every genre. What happens when a rock band plays a song in a distinctly non-rock style? Would you call “Honky-Tonk Women” by the Stones a rock song or a country song? Should there be a separate genre called “country-rock”? Would you call “Nothing Else Matters” by Metallica a metal song just because they’re a metal band? Should there be a whole separate genre called “power ballads”?
My friend Jon Oliver is a DJ, and he needs to organize his music into extremely specific categories depending on the crowd he’s spinning for. He’s evolved an elaborate filing system that’s based more on non-exclusive descriptive tags than exclusive categories. You can see an example here, click to see it bigger:
Pandora and Last.fm use tagging systems too, which I think makes much more sense than the genre category system in iTunes.