I understand that covalent-network solids are hard and have high melting points and that molecular solids are soft and have low melting points, but how do I determine the difference if given an chemical formula?
For instance, I was given the problem "Which of the following would form a covalent-network solid? NaCl, Ag, C, CO2, CuFe"

I eliminated NaCl, Ag, and CuFe immediately, but how do I know from looking at it that CO2 is a molecular solid, not a covalent-network solid like carbon?