I can absolutely guarantee that SLX was introduced in the 1983-84 model year. Prior to that there was only SL/SP.

If there is ribbing in the seat tube at the BB, it is SLX, if not, well...

I have seen many bikes with SLX decals that were not SLX.

You may be able to see by shining a light down the seat tube. The ridges are not large, but there will be five spiraled at the bottom.