As you can’t reply in the official topic, I reposted this in the unofficial area.
The two simplest ways to tell them apart is that:
-Aluminum is much lighter than steel
-Aluminum has more of a matter finish, and steel is shinier
If you want to be 100% sure or don’t have a comparison, just use a magnet. The magnet will stick to steel, but not to aluminum.
To add to that:
[LIST]
*]Aluminium is almost three times lighter than steel, so it’s really easy to notice the difference
*]Aluminium is a different colour, steel is slightly greyer
[/LIST]
also the aluminum is a softer metal so be careful when cutting it as it will bend. Also make sure your motors stay tightened or else the motor will shift and grind the axel along the aluminum. Had that happen on our drive this year 
Another thing I forgot is that aluminum is thicker than steel.
One method that is sure to annoy your (former) teammates is to drop the parts on a hard surface. Steel just clinks or thuds, while aluminium usually rings pretty loudly and clearly.
Repeating this technique on a large number of parts may drive the programmers insane. Use with care.
I just look at it, and if that fails me (which it never does) I pick it up 
It’s not hard to tell the difference.
2 methods I know of:
A) Set them both outside for a year. Whichever one rusts is steel.
B) File them both down into powder. Turn them into thermite. Whichever one ignites is aluminum.
Disclaimer: both will make the metal un-usable. Also the thermite will most likely burn at extreme heats. Neither method is recommended if you actually want to use the metal. 
Another good way to do this is to taste them. I’ve noticed that steel has much more taste than aluminum. Just give them a big nice lick and you’ll know.
Are you sure? I thought thermite was a Vex-certified part… there goes my design
Thank you everyone for your reponse will try some of those.
Probably the most easiest and most effective method is using a magnet.
Aluminum will not be attracted nor repelled; meanwhile, the steel will be attracted.
You should also be able to tell by how hot/cold they are to touch.
I suggest hitting them with a blow torch. The first one to melt will be the aluminum.
Or maybe try putting them on a grinder while your metal shop teacher is watching. You’ll know you have aluminum when he or she rips a strip off you for putting aluminum on a grinding wheel!*
More usefully, perhaps, steel will usually scratch aluminum as it is usually the harder material.
Jason
I think I’ll just use the magnet trick. My school would be happy if I was blow torching the vex metal.:p:D
If you hit a piece of steel against something it will make a different sound than aluminum
I have noticed that the Aluminum is a bit thicker than steel
To be blunt, Aluminum is a lighter gray, while the steel has a shiny finish. Also, Aluminum is a LOT lighter then steel, about three times lighter. You can feel the diference.
My students know when they have aluminum. They watch for the tears in my eyes when a piece has to be cut or when I’m clutching my wallet tightly.
I stare at them from across the room whenever one of my teams puts an aluminum bar on their robot that juts out of the robot. Cutting aluminum is like physical pain and I’m not even an adult paying.
All great suggestions. I was going to suggest something along these lines. I’d go to a metal smelting factory. Which ever one melts first is the aluminum.