Look inside
When field dressing, cut open the rumen and take a photo. Acorns, brassica leaves, clover, corn, soybeans and woody browse are easy to identify. Note the date and what you found.
Hunt the food they're actually eating
If every October rumen is full of white-oak acorns, your food plot isn't the destination yet. Track this over the weeks of the season and you'll know exactly when deer switch food sources. Then hunt the transition.