people do reload steel and aluminum cases. the downside is steel is hard on your dies, and aluminum is not strong enough for multiple loadings.
aluminum is easy to spot, it looks like, well, aluminum. Once you have seen one, you will pick them out instantly. if you are unsure, they are noticeably lighter than a brass or steel case.
For steel, a magnet will get them out. when I come home with a new batch of range brass, the first thing I do is put them in a container and swipe a magnet out of a hard drive (REALLY STRONG) though it a few times to get out all the steel.