You can know immediately if a food is good for you personally or not.

If you want to know if your body will benefit from eating something, do this before putting it into your mouth:
1.  Stand upright in a balanced and easy stance.
2.  Breathe in deeply and normally through your nose, trying to completely fill your lungs, throat, sinuses.  Don't strain; just take a nice deep full breath.  Can you breathe freely the whole way up?  If so, let go of the breath and go to step 3.  If no, breathe out anyway and wait for another day when your breathing is clear.
3.  Hold the substance in your hands.  Bring your hands up against your mid-section just under your rib cage in the solar plexus area.  Hold the substance gently against yourself with loose hands.  Relax. Clear your mind and try to get rid of ANY opinion about the matter.  You must want to know TRUTH.
4.  Ask yourself something like "Is this good for me?"  or "Should I use this?" (a simple yes/no question).
5.  Try again to take a nice deep breath through your nose.  Don't strain.  Just breathe in naturally and normally.  Do you notice a change?  Does it feel like your breathing muscles shut down a bit?  Does it feel like your lungs don't want to fully inflate?  Can you only take in a partial breath?  If so, your body is not compatible with whatever it is you were holding. 

Regardless of what any diet/nutrition/health book or article or show says, not everyone can or should eat the same things, and this is one way to isolate whatever is incompatible with you personally, regardless of how someone else responds to it.  It's kinesiology, or "muscle response" testing you can do anywhere, any time. 
Try it on food.  Try it on toothpaste, vitamins, shampoo, laundry detergent, spices, beverages, fast foods.  You will find that much of what you are using for cleaners, supplements, food, hygiene, room fresheners, beverages, etc. works against you and actually depletes your muscle strength.  Avoid those substances.