Sig no longer makes its slide assemblies with the older styled stamped/folded carbon steel. They are ALL made of milled stainless these days. If you have ANY doubt, look at the breech block retaining pin. If it is a solid one, the slide is milled stainless. If it is a split roll pin type, it is a carbon steel slide. Keep in mind though, that Sig NEVER made the P229 any other way than with a milled stainless slide. If there is a split roll pin then it was put there by someone that didn't know what they were doing or someone replaced the P229 slide with an older P228 one.
Regardless of finish, ALL p229 slides are milled stainless.