Stainless would polish up with a brighter lustre than aluminum if you use some metal polish.
Stainless would also be heavier than aluminum (but if you've only got the one keg, what would you compare it with?).
If you were to hit the upper handle ring with a grinding wheel, the sparks given off would be another way to tell if it's aluminum or not.
Some stainless steels (with a lower nickel content) would also be mildly magnetic. Try a magnet on it, and see what happens.
As for me and my household, we shall serve the Lord. Joshua
"God is great, beer is good, people are crazy." Some country artist