I know this might sound obvious to some of you, but you don't have to wash your whole body or head in a new product just to try it out. That could be really ITCHY and maybe even dangerous! Actually, PLEASE don't buy a brand new shampoo that you have never tried before and jump in the shower and scrub your whole head with it and let it get all over you without doing a simple little test first.
Take your new shampoo, and put a small dime-sized portion of it on your inner forearm. Rub it in with a Q-tip and leave it there without washing it off for 15 minutes. If it starts itching or burning before the 15 minutes is up, wash it off and put on some hydrocortizone or benadryl cream!! You are allergic to it and can now feel free to return the bottle to the store. If it doesn't bother you at all, you can feel pretty confident that the shampoo probably won't mess you up. That is, unless you have sores on your head or scratches. . . or get it in your eyes or mouth, that is a different case because your healthy skin can only absorb so much.
Also, some allergies are based on the quantity used. But the point is. . . start SMALL and don't have a huge allergic reaction blow up all over your body if you are trying new things.
Or, you can ask your allergist to skin test you in his/ her office with a new product. Just thought I'd state the obvious here because sometimes we need to be reminded of the basics of being an allergic person.