As you said you need good knowledge, however this isn't too hard. Just find the sport you know most about right now(however little that is) and watch like a whole season, or a whole bunch of games and tournaments(this obviously depends on the sport). Once you've watched enough you'll get a feel for who is good against who and bad against who. After that you're set, but this may take a while. It's all about just sitting and observing for a while.