In Chemistry how do you know the total charge of cations and anions of elements?

I have a chart but some of the elements don't show the total charge or 2 possible charges. For example, my chart shows Mg has a 2+ charge. But Fe could be 2+ or 3+? How do I determine which one? Also I can't find a charge for group 4A or 5A does that mean there is no charge and the compound would be covalent? Thanks for any help!