This is for desperate situations only.

Is it rotten?
If it is slimy or looks as if fungus or another type of parasite is eating it then don't eat it, you're not that desperate yet.

Bitter almond smell
Crush a bit of the plant and take a whiff, if it smells like peaches or bitter almond, discard it as it is most likely poison.

Skin rash
A good test, though one that may take a couple days, is to rub the plant on a section of arm or leg and see if you get a rash or whatnot. Rash = don't eat.

Cook
When you're initially trying to figure out if you can eat it or not, cook it.

Tasting the most likely poisonous substance
No rash, not rotten and it doesn't smell like poison? Cook it. Put a small bit in your mouth. Bitterness is a bad sign. Burning of the tongue and mouth is a bad sign. Feeling like you're going to vomit is a bad sign. If none of these things are present, eat a small amount and wait several hours. If you don't die or feel terrible and you're still desperate then, my friend, you have found something to eat. Increase portion sizes slowly.

If you feel a sharp pain, drink warm water and don't eat anything until the pain subsides. If you become very sick and are near a fire pit crush up some charcoal and eat it. This will make you vomit and help neutralize the poison.

Hey, good luck champ, you're gonna make it.

How to drink urine to survive
Wild plants you shouldn't eat
edible flowers
How to find north

Additionally, most wild animals only eat nontoxic plants, and most of these are edible for humans too. For instance, mushrooms are an excellent and often plentiful source of woodland food, but very perilous to eat unidentified. So if you don't know a death cap from a magic mushroom, you can often find a mushroom that is partly gnawed. If you don't mind sharing your food with a strange animal, this is safer to eat than your average shroom. Some of those many nasty looking shelf fungi you find on rotting logs are good food, and plentiful too. Fern are also very plentiful in most areas and generally not highly toxic, (in Taiwan some varieties are grown for food!) but some are highly carcinogenous, so don't eat unidentified fern unless you are really starving and prefer a slow death to a short one.

Other than that, only eat the sweet bumpy raspberries and blackberries you find--most smooth red or black berries you find in the wild tend to be poisonous or contain loads and loads of theobromine, theophylline, and/or caffeine (the first two are particularly potent)--so they are not a good food source either.

If you are in a marshy area, look for wild mint, catnip, and lemon balm--easily distinguished by the square stems and distinctive smells. They aren't a good food source either, but they freshen your breath after eating the shelf fungi and make a mean tea to boot. While in the marshy area, pull up all the cattails you see and keep the roots. You can peel them, dry them, and pound them to make a coarse flour (remove the fibers) and should you have a pinch of baking soda and a fire on hand, you could bake yourself some sivilized bread.

Wild carrot, or Queen Anne's lace, is a ubiquitous weed in meadows. They have a tough fibrous white root that may eaten, again a bad food source and with a very pungent flavor but you are desperate right? Don't confuse these for poison hemlock if it grows in your area! Look for the little black dot in the center of the flowers.

Then there's always the old staple of dandelion greens; best eaten young before it flowers, as it gets bitter with age. The jagged leaves are very distinctive, and it will make a nice salad with your shelf fungi, mint leaves, and berries.

The advice given to CCF cadets regarding eating wild plants is, as I recall:

• Rub a little on the the legs. The feet have thicker skin but if they become inflamed then you can't walk anywhere. Wait twelve hours. If there is no reaction, proceed to the next stage.
• Rub the potential food on the wrists. This puts any poison present closer to the bloodstream, providing a more sensitive test. If there is still no reaction, proceed.
• Put a little of the food on the tongue. Wait another twelve hours, for any reaction. If there is still no reaction, eat up!

You'll notice that this whole process takes up to 36 hours before you can actually eat anything. It is designed for you to begin as soon as you find yourself in a foreign environment, rather than when the food supplies are exhausted and the troops are starving.

The system is in typical British Army style: damage limitation. It'd have to be a pretty strong poison to take a man down just from contact with thick skin, and then the sensitivity of the tests are built up from there. The only risk would be if the test subject built up his own resistance, while the other troops would dive straight in after the trial period. That said, I don't know of any cases in which this occured.

Look at the sap (if sap is present) if you cut/break a plant stalk and discover a thin, milky sap then you've also likely discovered a toxic plant (dandelions are okay, bitter, but okay). Unfortunately, no one's ever told me what healthy sap should look like, so you're on your own there, sorry.

Seeing what other animals are eating is a good way of determining which plants are edible, but not always the best. Reindeer have been often known to eat various psilocybe (magic mushooms) at times, and seem to enjoy it almost as much as humans do. Most larger animals can break down some compounds much better than humans can, and while these plants probably won't kill you, they could make you very sick and if you're in a situation where you need to turn to wild plants for food, that's bad juju.
Of course, you may decide to go after the animal you were watching too, at the very least you'd get some exercise from trying to chase it down.