BTW... for most of the brew process, the welds don't have to be sanitary. You really only NEED sanitary welds for those in contact with the wort after cooling.
I have since bought a TIG welder and started practicing. I welded my first to couplings into my HLT to add a copper coil this week. Here is a couple pictures of a practice coupling I did:
Okay, here's another way to tell if it's stainless. Weigh a keg. It should be approximately 35 lbs empty. If it's aluminum, I suspect it'd be a lot lighter.