Some have said that you can tell with a magnet but that is not correct- some stainless steel is not only non- magnetic, it is not even considered a ferrous material even though the major portion of the alloy is iron by far.
They really do not look the same but either could also be finished with almost anything.
As some have said trying to abrade the corner will show pretty quickly which is which as almost all aluminum alloys are much softer than stainless steel. In the end though it is weight which is usually obvious; steels, including stainless steels, are 3 times heavier than aluminum at least as a 'rule of thumb'.
And finally few items are made in both of those materials. They are really not direct competitors and depending on the process, one will be much easier to work with. Aluminum is much easier to cast due to its much lower melting temp (less than 1,100 F compared to over 2,000 F for S.S.). Stainless steel is much easier to weld and <usually > form as long as you can heat it a bit.
But if you really cannot tell and absolutely must know then a small sample and a propane torch will tell you; if it melts, it is not stainless steel and may be aluminum or some alloy of zinc (if a really cheap object). If it does not melt and is not magnetic, is some version of gray then it most likely is stainless steel. To determine the exact alloy will take laboratory analysis though.
Brian
I would have though this was a stupid question untill today. I salvaged some scrap metal off a grill going into the scrap yard. But now that its home i'm looking at it. and besides weighing it, how do i know if its stainless or aluminum? No magnetic signature, and looks about the same.
I was thinking just put the torch to a corner and see if it glows?