* Check the sticker: Look for the original window sticker (often found in the glove compartment or owner's manual). This will have the EPA estimated MPG.
* Consult the owner's manual: Your owner's manual should list the expected fuel economy.
* Use the EPA's website: The EPA's FuelEconomy.gov website allows you to search for fuel economy information based on year, make, and model.
Without knowing the specific engine and transmission of your 2004 Chevy Cavalier, I cannot give you a precise MPG figure.