Beakerblog found the mother lode of all flint mothers this week. Some of the nodules are huge, Danish dagger worthy. This particular lot lode is weirdly like 90% flint, different colors, different qualities. Not exaggerating. There is literally several TONS (tons!) of it is this particular parking lot.
The best place to find flint is actually in the parking lots of offices and businesses that have decorative rock beds. A lot of time they truck in river stone. The best way to know for sure you've got flint is to strike it and it'll smell like a cigarette lighter.
Always ask permission to hunt for rocks, very especially on private lands, and probably the same for doctor's offices, banks and small businesses. My advice for the buy-n-large box stores is, well, that's for another blog.