Try the magnet test? Sticks = mild. no stick = stainless. What wastegate is it? If it's the 2-bolt flange for a 38 turbonetics/tial it will be mild. We had a limited production for the turbonetics v-band rg-45 in mild. Most of the downpipes for precision v-band pw46 were stainless.
hello boys; There is SS that will atrack a magnet. It depends on the pull of the magnet. If it grabs with force it's mild steel but if the grab is slight it's SS.
The TA headers I got does that and when I got them coated the pull explanation was told.