1. Take it before eating something suspicious. Try to rest 5 min before eating so you'll get your resting pulse.
2. Then, after eating, take the pulse again. You may take it also 30min, 60 min and 2 hours after eating. That way you'll get a curve.
With at least 2 measures (right before and right after eating) now compare the 2 values. If the second value has an increment above 15% it may indicate some adverse reaction. The higher the percentage of the difference, the more is the likelihood.