How could you distinguish an element from a compound?

If it is on the periodic table, it is an element.

Explanation:

An element is a pure substance and is found on the periodic table. If you have more than one element chemically bonded, it is a compound.

Examples
Hydrogen, oxygen, and iron are all elements.

${H}_{\text{2}} O$ is a compound because it is hydrogen and oxygen chemically bonded. You won't find a symbol of ${H}_{\text{2}} O$ on the periodic table.

Similarly, $F {e}_{\text{2"O_"3}}$ is a chemical compound made of elements.