Look for a date stamp on it (usually on the top section, where the valve is). It should be on a down diagonal from the brewery name imprint (above the valve opening) to the right. You'll see a fractional number like "14/18" then the gallon size, like "7.75" followed by two numbers. That's the year followed by month of manufacturer. Such as 96 10, making October 1996... From what I've been told, any keg made in the 90's (and newer) IS stainless steel.
A magnet won't be able to tell you if it's stainless or not, since the alloy used is non-magnetic (303 I believe).
As mentioned, a good metal file will start removing material from aluminum with little effort. Same effort on stainless will remove very little material.
Also, if the shape is like the current Sanke's you see, chances are it's stainless.