A function included in the GECK version 126.96.36.199.
Returns 0 for an undiscovered marker, 1 for a map that's visible, and 2 for a visible marker that can be traveled to.
if ( markerref.GetMapMarkerVisible == 0 ) ; Do stuff you want to do if marker is not on map at all endif
if ( markerref.GetMapMarkerVisible != 0 ) ; Do stuff you want to do if marker is on map endif
if ( markerref.GetMapMarkerVisible == 1 ) ; Do stuff you want to do if marker is on map but player can't fast travel there yet (player was shown map location but hasn't been there yet) endif
if ( markerref.GetMapMarkerVisible == 2 ) ; Do stuff you want to do if marker is on map and can be traveled to (player has been to location or marker was enabled as being travel ready) endif