Question behind the question: I was actually thinking about the implications of dark curses: Anyone can use a dark curse on anyone else (provided they are properly educated in magic and so on). For Avada Kedavra and Crucio, that didn't seem too bad. In America, people are allowed to own guns, and abusive people exist. Imperio seems problematic. Anyone can control anyone else? Wow. It's like everyone has a ring of gyges. Perhaps I am cynical, but I highly suspect people would use Imperio if given the chance since they would be unlikely to be caught, or so I assume. Thinking about it more, a lot more people would be dead or abused if their murderers or abusers had higher probability of escaping justice.
The somewhat dumb question: So now I, of course, question my assumptions. How would dark curse casters be caught? In general, how do aurors or anyone catch criminals, solve crimes, uncover mysteries or murder mysteries?
Side question: How do you know anyone is under the Imperius Curse?
I don't think spells can be traced aside from Prior Incantanto, and that's assuming you ever encounter the one who cast the spell.
I'm guessing wizards don't check for DNA or hairs because dark wizards/criminals would likely just say "accio (hair, DNA, evidence, etc)" or something.