You order a pizza to-go at the restaurant, and before you pay for it, you open the box to check the pizza, to make sure they got your toppings right and that they're not giving you someone else's order.
In a different example I wrote "The children are checking on their dogs" and it was marked correct. Here I wrote "I am checking on the pizza" and it was marked incorrect. ??
[I do like these sample sentences because they are like life -- sometimes surprising. ]
You can't tell just by looking. "Kosher" partly means "containing certain ingredients" and partly "has been blessed by a rabbi". It's a mystical term based on a combination of traditional dogma and superstition.