Which piece of evidence would suggest an unknown substance is an ionic compound?

Ionic substance is a compound that comprises elements that are glued together by a force called ionic bonding. Ionic boding is an electrostatic force between ions. 

Evidence:
Unlike metallic substances, ionic compounds (in it's solid state) do not aid the transfer of electrons. In other words they do not conduct electricity. It is not true when we talk about ionic compounds dissolved in water. In liquid form or in solution form, they now display conductive abilities unlike when it is in its solid form.