I know this question has popped up before, but what I have to say is nothing you have seen before! Bwah hahahaha ha ah! *coughs* Sorry about that. As you may know, The Joker is Batman's greatest foe. He will slice your throat for not laughing enough. He'll shoot your family if you crack a bad joke. He'll even detonate a nuke in Metropolis. But that's besides the point. Or is it.....
If you have ever seen Christopher Nolan's "The Dark Knight", then you will know how much of a psycho he really is. From robbing a bank, to blowing up a hospital. Where does his knowledge of explosives & what happens when you "start with the head" come from I hear you ask? Two words: military...experience.
How would The Joker know how to perform those^ drills? Like I said military experience would definitely allow The Clown Prince of Crime to know how to rig explosives, or torture (referencing to the scene right before he wants his phone call when he says "Do you want to know why I use a knife? You see, guns are too quick. You can't savor all of the little...emotions. In their last moments, people show you who they really are. So in a way...I knew your friends better than you ever did. Would you like to know which of them were cowards?" meaning he would use a knife to torture and/or kill people)
Now you're probably asking your screen "What about how the police or Batman don't know who he is!?" And to that, I give you this.
Notice what Knightwing says. How the Bat-Fam's DNA is protected (meaning that if they leave some saliva, blood, sweat, etc. anywhere, it can't be traced back to them.)
The Joker must of had his DNA wiped from any records when he was in some sort of black-ops group. Also assuming that he or somebody he knew was a highly experienced hacker.
Going back to what I said about the drills & torture, he would "take care" of terrorist, or a target having them tell anything & everything they know. He would have done drills when he was training in the military.
And he would know where to get explosives & detonators. His scars would of been from somebody maybe trying to slice up his face in a fight/battle. PTSD (post traumatic stress disorder) could explain his destructive & murderous behaviour towards people & things.