How do you know if an animal is a primary consumer or secondary consumer?

Yes, some can be. Some are considered to be 'vegan' and are only primary consumers. They would not eat anything but plants. The Giant Panda is an example. Some others that normally will eat only plants do eat other things like insects or even mushrooms at times, making them secondary consumers. Humans and other animals will eat anything and are omnivores. I guess they would also fit into being both.