Hi Michael
just to confuse the issue more...
my old ones were steel. They are DEFINITELY painted. The ones I bought off Craigslist which ARE stainless, and which I have polished, are also magnetic! How magnetic? Since i don't have a strain gauge I cannot tell the difference by feel alone.
The only real difference I found that is obvious on my old ones is slight rust spots at a few areas of the painted surface on the back (see pic). However, I am not willing to strip the chrome or paint off just to see, because the caps are in great shape everywhere else. The only (probably insignificant) difference otherwise, apart from the slightly different sheen of chrome vs polished stainless, is the weight difference- the stainless ones come in at about 630-640gm, the steel ones I have all around 700gm-this, I suspect is more a gauge of thickness difference than anything else, and may have no real meaning other than that. So. bottom line, I have no clue how to check them other than removing the paint chrome to see what lies beneath if there is no obvious rust. Even if you saw them side by side you would be VERY hard pressed to tell either.
(See attached pics. On the bottom, the one that is steel rests on top, thus is the lower rim)
Now, aren't you glad i am totally muddying the waters?