From my own LONG personal experience in identifying the culprits in my food I found the first thing to look for is any monosodium glutemate (MSG) it now does not need to be on the label here in the US, because if they call it a natural flavoring ingredient it seems to pass their "test" of concealment. It can play havoc on the entire body.

What most notice first is reoccurring headaches with no explanation, then that leads us down the path of misdiagnoses and deadly treatments that move us on from simple headaches to chronic fatigue syndrome diagnosis, on to a fibromyalgia diagnosis and later on to things like MS and Parkinson's. This is where due diligence must come in to play.

As @steeminganarchy mentions: Process of elimination and of course A LOT OF RESEARCH!