It depends on what type of food you are cooking and the method of cooking. Usually, when you fry food the color changes and that's how you know it is ready after a few minutes. Just follow the recipe and you can't go wrong.
I usually stick a knife in the middle of it and if it comes out clean, I know it's done. But that's really only if you're baking something. If it's actual food you're cooking, try cutting it in half to see if it's cooked through.