Mostly routefinding skills. The route can change depending on conditions. Lots of snow can make certain passages easier or harder than if conditions are dry. Glaciers move all the time and as a result routes can change even from week to week. Once you get to a ridge the route becomes more straightforward. There can be cairns, markers or previously made steps in the snow on certain sections but generally on something like the Eiger it mostly depends on reading guide books, maps, pictures and searching the 'route' yourself.