Roughly half of all plants growing on earth are edible. In general, identifying a few poisonous plants will allow you to avoid most of them by observing key traits shared by similar plants.
When in doubt, try the plant on an empty stomach. Touch the plant to your tongue and take a small nibble. Chew, but do not swallow. Observe the taste and texture of the plant and then spit it and any juices, including saliva, out of your mouth. Rinse out your mouth with water and again do not swallow. Do not eat or drink anything for several hours after experimenting. Observe the effects of the plant on your body during this time. If no ill effects occur within 24-48 hours, try eating a small quantity. Watch for ill effects and slowly increase the amount of the plant you consume over the course of a week. If you haven't suffered any ill effects at the end of the week, consider the plant harmless and edible.
"It is generally safe to try wild plant foods you see being eaten by birds and animals; however, you will find few plants of which every part is edible." - FM 21-76, Survival, Department of the Army Field Manual.