There are many structures and shapes that give hints at the function of the cell. The shape of single-celled organisms varies from multi-cellular organisms, and the shape of each cell within a multi-cellular organ vary depending on their purpose.
Some cells are shaped like small creatures and have organelles that specialize in helping the cell move. For instance, according to Grow magazine, bacteria can move using a tail-like flagellum or hook-like pilli. If a cell's shape includes structures like this, it is most likely a single-celled organism.
Other cells belong to plants, and these cells typically have cell walls. Cell walls are thick, somewhat rigid structures that surround the cell and give it strength, and they are not found in animal cells.
Finally, animal cells can be incredibly diverse depending on their function. According to Washington University, neurons, the cells of the brain, have branch-like dendrites, a long axon and more branch-like terminals at the ends, giving them the appearance of a tree. These structures allow the brain cells to transmit signals throughout the brain. Muscle cells, on the other hand, are long and tubular, allowing them to move body parts. The skin cells on the outermost layer of animal skin are full of proteins and fats; these protect the animal from bacteria and wounds and help the inner layers of skin to stay moist. As another example, red blood cells are shaped like discs with a large indentation in the middle of both sides, which helps the cell transport more gas to and from the cells.
Each cell has a specific purpose, and there are many more examples of structures and cell shapes that are an important part of a cell's function.