Identify the following compounds as polar covalent, nonpolar covalent, or ionic?

You can tell if a substance is ionic,covalent by looking at each of the individual atoms which make up the compound. Covalent substance is made up of ALL non-metal substances, and the atoms have to share electrons to bond and become stable. an  Ionic compound is made up of non-metallic and metallic substances,
As for the non-polar or polar bit, there is no way to tell from the chemical formula, although you can try out the substance by trying to dissolove it in water, if it does dissolve, its a polar substance and if it doesnt, its not.
Hope this helps:)

Look at the electronegativity difference for each molecule. You will need an electronegativity chart to do so. If the difference is less than 0.6, it is just covalent. If it is between 0.6 and 2.1, it is polar covalent. Anything above 2.1 is ionic. For example, the eletronegativity of Na is 0.9 and the e of I is 2.5; thus I2.5-0.9I = 1.6  so the molecule NaI is polar covalent.