Granted, it's not often we get the time to work on a story where these questions would come into play, but I thought this was great stuff from The Washington Post's Hank Steuver. He shared questions he asks himself to figure out whether he's spent enough time with his source.
Stuff like: Have I seen them with wet hair? Have I seen them shop? Have I seen the inside of their fridge?
Keep the list around (or the link to it). Might come in handy sooner than you think.