What kind of trick could you devise to find the person who has been stealing food from the lunchroom 'fridgerator?

 

The usual tactics:

1. rotten food
2. some type of ethnic food uncommon to the local palate
3. disguising a non-food item as food 
4. planting a medicinal substance in the food, such as a laxative 
5. a strategically-placed camera*
6. work it out with a few others or several others who have been robbed to be on the lookout for a particular meal item being eaten by a suspect
7. leaving bait food (as in an item that has frequently been stolen in the past), and combining it with any of the other methods listed above

I just realized that numbers 1 through 4 are more revenge-related or payback-related than actual discovery of culprits, because they don’t necessarily have the “gotcha” moment of exposing the person (s) involved.

*This one was used at one of my workplaces and it worked very well. As it turned out, the thief was a middle manager, and the ruse resulted in ending the misdemeanor behavior for good. 
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Tell everyone it's "white pants" Tuesday, lace the food with ex-lax, and lock the bathrooms.  The evidence will speak for itself.

Put a dye bomb in a container with the sandwich.
Place a large spider in the container.
Put a dead mouse in the container.

Put a lovely piece of poisoned cake in the frig and tell everyone other than the food thief. 

Laxatives.