Unfortunately, one can’t simply read the ingredient list on a food package and know if the product is kosher. Ingredients like glycerin, gelatin, and natural flavors all have both kosher and non-kosher sources.
Kosher consumers won’t buy a product unless they’re confident it’s kosher. Likewise manufacturers need a way to persuade kosher buyers to trust that their products kosher.
The solution to this problem involves independent kosher-certifying agencies. Producers pay these agencies to inspect their ingredients and processing plant, and certify their products as kosher. A variety of symbols are used to indicate which agency has certified a product as kosher.
For example, look at this label from a bottle of Heinz ketchup:
Just to the right of the pickle, and you'll see a U inside a circle, a trademark of the Orthodox Union, also known as the OU. The OU is kosher inspection agency. When consumers see that symbol on a product, they know that the OU has certified that product as kosher. The OU is the largest kosher-certifying agency in the world. There’s a good chance that you have several OU-ceritified items in your pantry.
Other well known symbols include the Star-K, the OK, and the Kof-K:
Kosher consumers look for those symbols and others to make sure the products they buy are kosher.