A SC A3 stock will have "FJA" and the crossed-cannons cartouche, no "RA". Early Rem A3's were the same, so you have to go a step further.

In front of the trigger guard, SC inspector stamps will be a circle, square, triangle, and diamond.

If you cannot see any stamps, look at the cut-out for the lower band spring. SC stocks will be rounded on the far right side. Rems are straight.