Best way to test for impact a food type has on you?

That's me, I've bought my shiny new meter all ready for the research to begin on what foods have what effect on me. And then I realised that I didn't know what was the best/most accurate/most useful way to go about the testing!

So would I be better doing:
a) Test, eat a known amount of just the food I want to assess, then test again at appropriate interval?
b) Eat a meal as normal (so effect of entire meal is known), testing before & after at suitable intervals
c) Both the above (on different days obviously!), so I can compare results.

Any other hints and tips for working out my ideal eating plan welcomed (as are links to threads with good info, doing a search seems to kill my work pc )