Yes, the Marrow_Sun and Mystery Helmet are essentially the same helmet, and as said above, the Mystery Helmet was produced in resin and the Marrow_Sun in fiberglass. Though, I find the Marrow_Sun helmet to be slightly sharper and in better shape the Mystery Helmet.
Unfortunately, there is really no clear indicator of a recast helmet. There is a fairly good chance that outside of a helmet produced by either Sarge or Marrow_Sun the helmet could be a recast, because they are the only two I am aware of that have found the original Mystery style helmets. However, some general tells of recasting include a suspiciously cheap price, soft features/ poor sharpness, size difference, and unusual warpage.