How can I tell if the air vent coming through my roof from my toilet is pluged with leaves or something?

Spock (rhp)

run a snake down it from the roof. in a single story house it is usually fifteen feet or less high

ricky

Same way as a water line, use a snake.

Rick

Well, your first indictor that it is not clogged would be that it smells like sh...unpleasant. Anything that can help you determine air movement when held right over the pipe like alit cigarette or stick of incense glowing brighter is an indicator for a working vent. A toilet that has an unexplained lower bowl water level or a sink trap that has been sucked dry (there's that odor again) is an indicator of a clogged vent. Very basically jam a stiff wire down the thing and see if you feel a clog. That's the easy stuff, a plumber is next.