The modern cent weighs 2.50g and is composed of 97.5% Zn and 2.5% Cu by weight. There should be 2.4375g of Zn and 0.625g of copper in a normal minted coin.
If it weights anything below 2.5g, it is probably unplated zinc. If it weighs 2.5g and your scale doesn't go to 100ths of a gram, you don't really know for sure. If it weighs over 2.5g, it is very unlikely to have been struck on an unplated planchet.