Insurance fraud investigation--trying to outsmart liars and cheats looking for a fast buck by scamming the system--may not be very glamorous, but these days in Pittsburgh, it's food on the table for P. 1. Carroll Dorsey. And with the economy headed south, fake fall-downs have turned into something of a working-class growth industry. Thing is, though, the more Dorsey looks, the more he sees. This phony-injury epidemic is bigger than it seems, and it might involve some of the heaviest hitters in town. If Dorsey makes the wrong move, he's liable to take the big fall-down himself and he's got no insurance. . .