What properties of a compound would lead you to expect that it contains ionic bonds?

The most definitive test would be the ability of the liquid phase of the compound to conduct electricity easily. (The ability of an aqueous solution of the compound to conduct electricity would be less reliable, because many covalently bonded compounds, such as the hydrohalogens and the oxides of sulfur, ionize spontaneously when dissolved in water.) Another indicator of ionic bonding would be a high melting point temperature for the compound.