I've never found knowing the "root" of my stress to be much of a challenge. I know what's going on in my life, know which things are stressful to me, and know which one's aren't. I don't need a biological marker in my hair to tell me if I'm under chronic stress.
It think - if people are honest - they'll say the same thing.
I know this doesn't answer the question; but I guess the answer is, "Consider what's going on in your life - and you'll usually find the cause of your stress there." :)