I'm thinking about american football in particular, but this question could be applicable to other sports.
When I'm watching a game on TV I've noticed the referees are typically very good about spotting penalties. I'm able to notice after the fact on the reply, but only in slow-motion and when the announcers tell me what I'm looking for.
Do the NFL's (or other sports') referees learn or use specific techniques that enable them to spot penalties easily?