I'm assuming you know what their icon looks like right? They are indistinguishable from adventurers. Other than that, on the world map, go to higher places to see units from farther away. In particular, mountains > hills > lichenous forest > heathland > coniferous forest > spruce mire > pine mire > open mire which is the same as open fields.
Also, some places provide cover to units on the map, the aforementioned coniferous forest and spruce mire tend to do a good job of hiding units on the world map, although not always. Bigger groups like robbers tend to be easier to spot. Also, your eyesight plays a big role in spotting units on the world map.