I guess my response to this question has developed rather organically (no pun intended).
It started with chafing under the arms, as a reaction to the stuff in laundry detergents. I stopped by a local health-food store, asked employees, checked out ingredients, talked with friends. I don't always buy everything at the health food store, but I'll usually start buying something there, at least once or twice, until I know whether I like it or not (like the laundry detergent, my body will tell me), and ask around of friends, massage-school classmates, online, and trusting my intuition (it usually doesn't steer me wrong).
I've learned that there are some companies I trust, and some I trust for certain things only...
-Tom's - great toothpaste
-The Thai Stick - deodorant
-Seventh Generation - laundry detergent, organic paper products, dish detergent, all purpose cleaner, almost anything they make
-Simple Green - anything they produce
-Dr Bronner - soap, unfortunately their shaving cream is too watery for my face
-Dagoba, Green & Blacks, Rapunzel, a few others - chocolate
Tanners Creek - arnica salve/massage cream
Jade & Pearl - shea butter/massage cream
Summary
-intuition
-references & referrals
-online research/education (a degree in chemistry helps)
-the people at health food stores/farmers markets/other experts
-online search
-compare to companies I've learned to trust
-try the product, my body will tell me if it doesn't like it