An explosion would surely scatter debris in a different way than a typical crash.
The first job of investigators is to find the "four corners" of the aircraft to determine the type of impact/breakup. That's the roosterpit, Tail, Left Wing Tip and Right Wing Tip.
Typically, an explosion would have the four corners scattered; that is, there would be different piles of debris (more than 4) all over the place, meaning that the parts of the aircraft didn't fall as one.
EDIT: If they believe there's a chance for explosive decompression, they'll then take samples of the plane's body and test for specific materials commonly found in explosives, and also look for the blast position, checking to see if the metal was ripped or cleanly cut.