Hi, Cheyenne. As far as looking at them on the outside, someone else may be able to come up with something. However, you can tell easily by their teeth which are which.
The arrangement of teeth in the oral cavity is one of the major differences between carnivores, herbivores and omnivores. Carnivorous animals tend to have wide mouths in relation to their head size and have highly-developed jaw muscles called the temporalis muscle. The teeth of carnivores are long, pointed and widely spaced to prevent debris getting caught. Herbivores have smaller mouths and muscular tongues to move food about and grind it down on the flat platforms of molar teeth in the mouth. Omnivores tend to display elements of both types, using flat molar teeth to grind food down and using sharp incisors to tear off chunks of food like a carnivore would.
The teeth is probably the most obvious thing, but it's not surefire. The main other difference is their stomach and other parts of their inner digestive system. Some may be actually unable to digest the other food, while others need their respective food absolutely to survive, because their digestive system cannot absorb certain chemicals in the other food that are necessary for survival, even though they can eat it and digest the food itself.
Usually their teeth because carnivores will have large sharp and or jagged teeth. Herbivores have all molers like we do to chew leaves and omnivores have both like we do. So we are considered omnivores.