What is the difference between stainless steel and aluminum?

Stainless steel and aluminum are almost indistinguishable... That is unless you know that some stainless steel sticks to a magnet.

Also, most stainless steel is non-magnetic, like aluminum. So how to tell the difference then?

The truth is the you can easily distinguish between stainless steel and aluminum by using the metal proporties. So for example if you have many cookware pots and need to know if they are aluminum or stainless, you could try a magnet. If they both are non magnetic, then try making one of them spark.

How To Do The Spark Test

The spark test is one of the most useful tools a person can weild when working in a shop environment. Weather you are welding, milling, analyzing, or scrapping many if not all of your decisions will depend on the exact type of material you are working with. That is why utilizing the spark test is important. 

The spark test is good for use on steel, tungston steel, carbide, chromium steel, stainless steel, nickel, nickel steel. The sparks that these metals shoot when ground with a bench grinder during the spark testing process, will be idiosyncratic to that material. I found a great review of the spark test here!

Spark testing is one of the primary ways to identify scrap ferrous metals. For more, check out more at scrap metal identification. 

The more metal you plan to come across, the more and more you will need to look into leasing a XRF analyzer gun.