If in doubt whether a frame is steel or aluminum, there a very simple test you can conduct - assuming the bike has wheels on it. Place the ball of your foot on the center of the bottom-bracket on the cranks. Holding the bike by the stem and seat - push. A steel frame you will feel flex and spring back. An aluminum frame will hardly budge.
A magnet off your 'fridge can also be used. It will stick to a steel frame.