Vegetables come in many different shapes and forms. Different parts of the plant are edible for different vegetables. In the case of potatoes, tomatoes and rhubarb only one part of the plant is edible, the rest of the plant is toxic. Eat only the tubers of the potatoes, fruits of the tomato and young stems of rhubarb. Leafy green vegetables are some of the easiest vegetables to identify because the plants are leafy and green.
Examine the fruit of the plant. Tomatoes are usually red and round and between 3 to 4 inches in diameter. Pepper fruits are red, yellow, orange and in rare cases, purple. They feel light for their size. The fruits are usually block shaped. Eggplant has purple fruits growing up to 6 inches long. The plant grows to 3 feet high and has broad leaves with a purplish cast.
Look at the roots. Carrots have long slender orange roots from 4 to 12 inches long depending on the variety. Radishes have round roots about 2 inches in diameter that are mostly bright red with white at the bottom. Parsnips are long and look like carrots but the roots are cream colored. Turnips are large, up to 4 inches in diameter. The bottoms are white and the tops of the roots are purple. Beets are dark red and round and can grow as large as turnips.
Determine how the plant grows. Beans grow on vines up to 6 feet tall. The vines twine around the support. Bush beans grow to 2 feet high and don't twine. The edible part of the plant are the pods for string beans, sometimes called snap beans. The pods of most other beans such as black beans, lima peas and fava beans are not edible. The seeds inside the bean pod need to be removed before cooking. Peas grow on vines that have tendrils that grasp the support rather than the plants winding themselves around the support.
See how the plant is shaped. If the plant grows to form a head or round grouping of leaves shaped like a rosette it is most likely either head lettuce, cabbage or cauliflower. Broccoli grows upright on thick stems. The edible portion are the immature flower buds at the end of the stems.