How can I differentiate between Ionic compounds, molecular compounds, and acids given only the formula? I need to be able to find the difference between an Ionic compound, a molecular (or covalent) compound, and an acid given only the chemical formula (no charges or roman numerals) ?

You can differentiate between ionic, covalent and molecular compounds by the fact that ionic compounds contain elements that include both a metal and a nonmetal. Molecular compounds contain both non metals covalently bonded to each other. While acids most often on their chemical formula start with the element of Hydrogen - H.

For Eg - sulphuric acid

H2SO4

hydrochloric acid - HCl.