Aluminum is a light metal that is used to make a myriad of household and industrial items. Many people recycle scrap aluminum for money, but in order to do so, you must be certain that your metal is actually aluminum.
Test it with a magnet
Aluminum is not magnetic. Bring a magnet around aluminum and it will not stick; it will simply fall off. Test your unknown metal for magnetic pull. If it doesn't react, then it could be aluminum.
If you have access to an angle grinder, try grinding a portion of the unknown metal down. Unlike most metals, aluminum will not produce sparks when ground with an angle grinder.
Estimate its weight
Aluminum is a very light metal. If your unknown metal is incredibly light, it might be aluminum.