Take the difference of the electronegativity between the two elements. The 50% ionic/covalent mark is a difference of about 1.7 (books differ and I see 1.8 or 1.9). Over 1.7 we call it ionic; below 1.7 we call it covalent. That is, a difference of 1.5 actually is about 60% covalent/40% ionic but you get the idea.
