Stop in to your local Petco. Pick up some expensive merchandise. Wander around a bit looking for the day manager. Ask them when the next $1/gal sale is.
Scurry around re-shelving merchandise that you were holding so you looked like a real customer instead of a bottom feeding bargain hunter, like many of us.
Seriously, ask the manager. They get the ads in advance and can tell you what's coming, if you ask. They don't always get the ads a long time in advance though. If you see a pile of tanks, odds are the sale is in the next month.