My guess on why you think you'd get closure if you knew which drugs, is there would be an element of control. At least you'd know Something for Certain in this crazy disease called addiction.
I too have an addict brother. Goes from alcohol to crack (still don't know which year he went from Coke to crack) and did steroids (heard he's off now) for years. I'm working on peace within myself and no contact. I just keep reminding myself that he's not a reasonable, rational or kind person anymore. Good luck and Hugs to you! Take care of YOU -- first and foremost!