A quick look at Wikipedia tells us that most of the 747 hull losses over the last twenty years are freighters. At least two of the freighters were disastrously badly loaded.
I'm unsure what two 747 freighter crashes you are referring to (I'm not finding any on a quick search), but if one of them was the National 747F at Bagram, that was directly caused by improperly restrained cargo that tried to exit the aircraft when they rotated - not 'badly loaded' (they actually found pieces of aircraft and cargo left on the runway as a result of the cargo punching a hole in the fuselage after it broke loose).
As TowerDog notes, there are onboard load sensors available for the 747F (I know some 747-8Fs are so equipped, although I'm unsure if it's basic or an extra cost option).