A lot of sitcoms are filmed in front of a live audience. You can usually tell which ones have canned laughter and which ones are legit.
Really funny shows like The Office aren't filmed in front of a live audience, but they're really funny so they don't need it anyway.
I think sitcoms have the canned laughter, and normal comedies don't.
Every episode of Friends is filmed in front of a live audience. Every laugh is different so you can tell it's not a laugh track...with a lot of shows there's usually some really obnoxious laugh that keeps playing at the end of the laughter.
/over analysing.