First, determine if it is a prokaryote or a eukaryote. Eukaryotes have a clearly defined nucleus with a nuclear membrane. Prokaryotes have only a single cell membrane.
Determine if the cell has chloroplasts. If it does, it is some sort of photosynthetic organism. If it has a rigid cell wall, it is a plant cell. If not, then it is a form of green algae.
Count the number of nuclei in the cell. If there is more than 1, it is either a fungal cell or the muscle cell from an animal. If the cell contracts, it is a muscle cell.
Finally, determine if it has chromosomes in pairs or just single chromosomes. Cells which have unpaired chromosomes are sex cells called gametes. They can be eggs, sperm or pollen.