We used to check the type of steel by grinding and looking at the type of spark emitted.
I would have thought it would also show if you heated the steel,a little like galvanised steel which gives off a dodgy gas .
Mick
Cor-Ten steel, once weathered, is obviously different in appearance when compared to weathered regualar steel. Walk out into the street and look at one of the steel man-hole or drain covers. Many of these, but not all, are of a rusty colour, but not flaky. They are also stightly smoother to the touch than rusty regular steel, and the rusty colour is darker. This will give you a clue at to the difference you are looking for.