I have a Centurion Le Mans. I estimate it's around early 90's: 7 speed indexed shimano sport LX shifters, freewheel, 126mm rear spacing, Tange 2 tubeset, biopace rings, 700c wheels, and non-lugged fork.

There has been some cable rub against the headtube to reveal little patches of shiny metal underneath. Being somewhat obsessed with shiny things, I love the prospect of an all-chrome frame. I read that some frames have chrome underneath, and some just have normal, somewhat-shiny steel tubes underneath. This frame has no visible chrome on the fork, stays nor lugs. The patch of rubbed-away paint has not oxidized at all in over a month (and I have since replaced my cables with longer housing, so it is not being rubbed away by ). I suppose this would indicate chrome, not straight steel. I realize this isn't a particularly high-end frame, but I hope it is still possible. CrMo does have the Cr in there, after all...

In any case, how would I know if it is chrome or not? If it is chrome, is it as easy as sandblasting the paint and maybe spray on some clearcoat somehow?

Some frames may be all chrome, but the high-grade polishing may have been confined to those areas intended to be exposed. IOW, the painted areas may not be as well finished as the unpainted areas.