I don't have the book in front of me, I borrowed it from the library and seriously want to buy a copy for everyone I know. I found it so helpful with my 3 year old daughter and wish that *someone* had clued me into the possiblity of food reactions long ago.
That said, it took me 2-3 times of checking it out and reading parts before I got anywhere with it, there is a lot of information there. As I recall, once you're pretty sure there are reactions going on, there are two options;
1) total elimination diet (outlined in detail in the book, automatically excludes just about everything 'reactive')
2) keep a journal of food eaten and 'meltdowns', to try to establish correlations.
When I started this process (last Oct.?) I wasn't entirely convinced, so I started with the journal as it was least intrusive. It took me a day and a half before I figured out one of the main causes of my daughter's outbursts - kiwi and melon.
It took me a little while longer (until recently, really, because I foolishly kept re adding it) to accept that dairy is also out. She simply is not toilet trained when she has dairy - she pees herself within half an hour of having had it.
Recently, I've been suspecting soy for causing grumpiness, but I'm not positive yet. It just doesn't come up enough and I'm not willing to experiment at this point.
So - that's my long winded way of suggesting that you start writing down what your child eats and noting behaviours. I believe that the book states that most reactions happen within 30 minutes (possibly an hour?) of having ingested something, making it easier to pinpoint.