How can you tell if a chemical compound is ionic or covalent by just looking at the formula?

Answer: Ionic compounds (Metal + Non-metal), Covalent Compounds (Non-metal + Non metal).

Explanation: Going by the definition, an ionic compound is formed when a chemical reaction occurs between a metal and a non-metal while a covalent compound is formed when a chemical reaction occurs between two non-metals.

Having a good knowledge of the periodic table, one can easily identify the metals and non-metals in each compound and thus tell if it is Ionic or Covalent.