The
long distance trails map doesn't show which routes are waymarked (most are, apart from Cape Wrath Trail and Skye Trail), if they show up as red it means you have yet to complete that route, if they are blue you have completed it.
The Scottish National Trail is made up of a number of pre-existing trails, most - but not the Cape Wrath Trail - are waymarked, you'd need to look at the detailed description for each of the trails to see more info on waymarking. There isn't any specific waymarking for the Scottish National Trail except a plaque on the Water of Leith and similar at the end at Cape Wrath. Hope that helps.