How can you tell if a compound is ionic or covalent?

Answer 1

An ionic substance will typically consist of a metal and a non-metal, such as AgCl.

Covalent molecules consist of non-metal atoms, such as CO2

The metallic substance in an ionic bond will donate it's electrons to the non-metal in order for them both to become more stable by having the same electron configuration as their neighboring noble gas. (Have 8 valence electrons.)

As the name suggests, covalent bonding involves the sharing of valence electrons to be stable for the same reason as above.