Is there dna in our food? how do you know?

Yes. Most food is organic based. That means it is the corpse of something, whether it be ant, animal, fungus, or in some cases even some prokaryotic creatures (but that makes it confusing so you can ignore that for now). All living things have DNA their corpses still contain the DNA for a while. There is an experiment you can find online where you pull the DNA out of a strawberry and you can actually see it.