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.