Hit with a magnet. If it sticks, it is not aluminum, but rather 400 grade stainless steel, or just normal steel.
The only elements that stick to a magnet are nickel, cobalt, and of course, iron.
As an aside, tungsten carbide doesn’t stick to a magnet. In fact, tungsten carbide is actually a powder. What is commonly called “carbide” is actually cemented carbide, which is cobalt impregnated with tungsten carbide (WC). The cobalt is what causes carbide tools to attract a magnet. (I have written about scrap carbide before)
If you grind the steel with a grinding wheel, and it throws off a glow of sparks, then it is steel. otherwise it isn’t steel. So if it is non-magnetic and it gives off sparks, then it is most likely 3xx series stainless steel.
Aluminum doesn’t rust. Stainless steel CAN rust, and contrary to popular belief WILL rust eventually, especially in extreme conditions.
Grab a similarly sized chunk of regular steel. Aluminum is about 3 times lighter than stainless steel. If your sample doesn’t seem light compared to the regular steel, then it is probably also steel.