How do I test if a fruit or anything else is edible?

Watch and see if the fruit or plant is being readily eaten by other mammals, like pigs or rodents. Look at it carefully and smell it. If it has a pleasant aroma and looks inviting, that's a good sign. Take a tiny bit and rub it on your skin. If it doesn't burn or cause irritation, move on to your lips, then your tongue, then finally eat a small amount. Give your body time to react to the fruit and if it doesn't make you sick, try eating progressively larger quantities. In general though, you should never eat any food that you're unsure about, especially when it comes to berries. Only resort to the above method if you must.