if someone were to spit in my food at a restaurant, how could i get the evidence?

[cat xo]

i know this girl would spit in my food, and i think it would be fun to get her back.. dont ask. but how can i prove it besides saying oh that looks like spit to me
well thanks for your thoughtful answer
good idea sarah

 

[Slippery Pete]

id spit in your food too. or maybe i already did !

 

[Sarah]

say you saw her??

or that one of he coworkers told you she did?

 

[Manny]

dna test that thing