How to Tell the Difference Between Stainless Steel and Aluminum

Magnet Test:

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.

Contrary to popular belief, tungston carbide doesn't stick to a magnet. In fact, tungston carbide is actually a powder. What is commonly called "carbide" is actually cemented carbide, which is cobalt  impregnated with tungsten carbide (WC). 

Spark Tets (Hit with an angle grinder):

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.

Rust Test:

Aluminum doesn't rust. Stainless steel CAN rust, and popular to popular belief WILL rust eventually, especially in extreme conditions.

Density test: 

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.