Fixing cracked concrete can only be decided case by case, but some rules to help you understand what you are working with is whether or not you have a structural or aesthetic crack. If a crack has spread to where you can slide a quarter into it, and or if the concrete level has changed from one side of the crack to the other then you likely have a structural crack. If you do not have either of these options you likely have an aesthetic crack. Structural cracks usually can only be fixed by removing the concrete (usually so the underlining earth can be fixed) and then replacing the concrete. Aesthetic cracks are less straight forward to fix as most of the time the methods to fix the crack require opening them up (and thus making them more obvious) to install the repair material.