Im not sure if the serial Number would show it as a DX or not. But A dash 4 would have the hammer mounted firing pin and forged parts. Also the DX guns had DX on the Bbl, so if it doesn't id be suspicious that its a DX. Hope this helps.
I have a 629-s Classic but not a DX. I'd rather have a 629-4 Classic DX but the one I have shoots very accurate. I have nothing to complain about except maybe that I can't shoot 300 +P+ rounds all day without doing damage to the revolver. But I figure a 250 gr. round can take down a bigfoot and nothing is bigger than that around my area. Black bears aren't nearly as big.