All Clad copper core unofficially induction compatible?
Although sales literature says no, I have came across copper core cookware with a little sticker added that looks like a spring and it is magnetic. So that should work with induction.
Are all of the copper core line like this or only some of them?
All Clad recently, within the last two years, started making Copper Core cookware that is induction compatible. This was done to address the growing number of customers in the US, and the predominant number in Europe, that use induction stoves. It are two easy ways to tell if the Copper Core is induction friendly. One, do the magnet test. Better, induction compatible Copper Core is marked on the bottom of the cookware with a very small R surrounded by a circle, just after the name of the cookware etched in the center of the cookware. When I say small, it is tiny.
When I bought my 10" skillet there was another one on the shelf. Mine did not say "induction" on it but the other 10" skillet did. Both were considered 2nds, & I took the non-induction one 'cuz it was a better 2nd than the other pan.
As the others have mentioned, take a magnet to your pan to find out. Some stainless is magnetic while others are not.