CP: I don't know much of anything about IN. But in that creek it was obvious. plenty of limstone around and all these dark colored chunks mixed in. You just gotta start picking through it and hauling out what ya want. If it flakes then take it home, some may be crap but some will work. Leapingbare's right: Flint is going to be denser that the surrounding limestone or whatever and you'll develop a "feel" for it as you go along - it's always heavier unless your in iron ore country, hehehe.
Jeff: Evansville is on the right track, just don't know if it's in that particular spot. Southern IN from what I've learned.. further East to be almost exact.