The simple rule I was taught that if you can find anything that moves, cook and eat it before eating unknown plants. Your odds are generally much better with animals than plants -- you can probably find dozens of examples of inedible or poisonous plants on your back yard alone, while poisonous (as opposed to venomous) animals whose poison is still dangerous after being cooked are quite rare.
An even simpler rule is to start with anthills -- ants live almost everywhere, and while many ant species use poison as a weapon, none are poisonous after cooking.
A caution here is that some fish, especially in tropical areas, are poisonous. Any mammal or bird can be eaten, some reptiles are poisonous though - box turtles, for example, can retain poisons absorbed from their food.
An even simpler rule is to start with anthills -- ants live almost everywhere, and while many ant species use poison as a weapon, none are poisonous after cooking.