Many wizards have magical ancestors, which is a clue that you might be a wizard but not 100% because you might be born as a squib. However, this  kind of apply's for muggleborn wizard because their ancestors are squibs and squibs come from at least one magical parent. And if your born from at least one magical parent or two magical parent, it's more likely that your a wizard but not 100% as you may not have inherited the magic.