This is a very good question! There are several ways, indeed first we have to watch what is going in on the field, we spend hours outdoors watching the animals. We also use camera traps to take pictures of animals approaching a plant to eat for example. Also, we can use colour powders in flowers, and they catch bats, for example, to see if their mouth has colours on it. Or, we can make some animals vomit to find out what they eat just before. There are a lot of techniques so we usually use a mix of those depending on the animal.