Never eat plants with thorns.

Steer clear of plants with shiny leaves.

Don’t eat mushrooms. Many are safe to eat, but many are highly toxic and even deadly, so it’s not worth the risk.

Umbrella-shaped flowers are a bad sign. Stay away from these plants.

Don’t eat plants with white or yellow berries.

If the plant’s sap is milky or discolored, leave it alone.

Avoid beans or plants with seeds inside a pod.

If it tastes bitter or soapy, spit it out.

Avoid anything that smells like almonds.

Same as poison ivy, stay away from plants with leaves in groups of three.

This list is from an article about how to test whether a plant is edible on How Stuff Works.