There are strategies for this, but the best plan is probably to learn a little bit of botany. Botany in a Day uses plant families to give you the chance to know basic knowledge on the most common plants in the world, wherever you are. I would start with a basic knowledge of botany, so that you can begin gathering edible plants before you begin the risky process of testing unknown ones.
For fruits in specific, you can get some idea of the edibility or effects by watching the actions of local mammals. If they eat it, it might be worthwhile to test. The eating habits of birds are nearly useless, as they can easily and happily eat berries that are completely poisonous to us. Also, some edible fruits will still give you digestive problems in excess.
If you decided to try an unknown plant, even ones bears or racoons were eating, I'd use the methods of the Universal Edibility Test(details below in sources).
That said, insects are typically a safer and easier to collect calorie source.