For the most part, fungi acquire nutrients from decaying material. Plantae Plants are multicellular and most don't move, although gametes of some plants move using cilia or flagella. Organelles including nucleus, chloroplasts are present, and cell walls are present. Animalia Animals are multicellular, and move with the aid of cilia, flagella, or muscular organs based on contractile proteins. They have organelles including a nucleus, but no chloroplasts or cell walls.
Animals acquire nutrients by ingestion.
Suppose you see something in freshwater that certainly appears to be living. Could be a plant or a protist, or blue-green bacteria. Make sure that the green is really part of the organism, though. An animal might have eaten something green, for example. Look for cell walls, internal structure.