How can you tell what an animal eats by looking at its teeth?

Animal teeth are the means by which an animal obtains and first processes its food.

Therefore if your food is grass your front teeth have to be able to grip and 'crop' the grass. In many cases this means that the top teeth disappear and are replaced by a rubbery pad which can grip the grass between this and the lower teeth. Then the back teeth have to grind up the cropped grass so that it can be digested so the back teeth are flat and broad.

With rodents the teeth have to gnaw at things so they have evolved to continuously grow.

With meet eating animals the front teeth have evolved into fangs which can grip and kill prey and the back teeth are small, and angled like sharp scissors to cut the meet off the killed animal.