All you have to do is go out to your garden, find stuff that has flowered, and watch a bee work to come to this assumption on your own as one will methodically visit every flower (often of an individual species), sometimes spending less than a second at each to determine if it has pollen or not, collect all it can while showing an impressive mastery of spatial awareness (while flying) and then return home with its payload.
They don't like to mix different types of pollen together in the hive, either. If you inspect a frame with pollen in cells, it's usually this cool array of different yellow/orange/brown colors because of this.
