Prepare the sample of cells. The best microscope for seeing a cell's individual organelles is a high-resolution compound microscope. The sample should be stained with a special staining liquid, which will help highlight the individual organelles. The staining process varies depending on the size of the sample; eosin is a type of stain that is particularly good for seeing cell organelles.
Look for the nucleus. The nucleus is what makes a cell eukaryotic. In a plant, the nucleus is clear and looks like a fried egg.
Observe the cell walls and chloroplast. These two organelles are specific to plants in eukaryotic organisms (though some prokaryotes have them), thus if the cell has all three of these organelles, it must be a plant cell. The cell wall is a permeable divider that lets water and other nutrients in and out of the cell; if you are looking at multiple cells, it is the clear dividing point between two cells. Chloroplast is the organelle that captures sunlight so that the plant can make food via photosynthesis; it looks like little green bubbles in most leaves.