How are Scottish hiking trails marked?

The short answer is that paths are rarely marked well, and it is generally necessary to have a map and to know how to read it.

Scotland has a long-established right to roam, meaning you can walk anywhere as long as you avoid cultivated land, gardens, and area immediately around buildings. That's great for hikers, but the flip side is that there are few formal rights of way, because none are needed. And if there's no formal right of way, there's generally no signage.

Generally mountain paths are not marked anywhere in Britain. Even on the most popular tourist routes up mountains there are few signs or waymarks, other than perhaps to get you across the fields to the start of the normal ascent. With the possible exception of the tourist route up Ben Nevis or Cairngorm, where the paths are so obvious it's hard to get lost, a good map (and not just a guide book) is essential; even on those two mountains, a map is still a jolly good idea. I recommend the Ordnance Survey series of Explorer maps, together with a compass. When the cloud descends, it's easy to lose track of which direction you were heading in.

Some long distance paths, most notably the West Highland Way (WHW), are signed reasonably — at least by comparison to other British paths — and in farm land or villages you can usually expect to see signs when a path leaves a road. These signs often only use the symbol for the walk, for example a thistle in a diamond for the WHW. But by themselves these are not sufficient to follow the path. You still need a map or a good guide book with maps.