TESV: Skyrim - Show all map markers cheat
open the console by pressing ` (or ~),
type
tmm 1
to toggle all map markers shown.
To toggle all map markers and still be able to "discover" them (aka not being able to fast travel to them, but they are marked on the map and mimimap, type
tmm 1
tmm 0
tmm 1
in that sequence.