This quest will appear in your journal shortly after you complete the "Smoke and Ashes" quest and step outside the walls of Laslea Glen - you will see one of the scouting towers right in front of you.
I noticed a tower nearby. Maybe I'll spot something worthwhile from the top.

1. Reach the tower
The tower looks like a fine vantage point. It shouldn't be hard to reach.
The tower entrance is guarded by three soldiers who have set up camp nearby. You can kill them, but it is not necessary.

2. Climb to the top of the tower
I've reached the tower. Now - how to get to the top?
If you don't want to fight the soldiers, you can go around the tower. On the other side, there is scaffolding you can climb using the Shadowstep and Planeshifting skills.

Keep in mind that this consumes stamina, so pause on the wooden platforms to let it recover.

Check the contents of the chest on the top level of the tower - inside, you will find a random Witchcraft Manual.

3. Scout the area from the vantage point
What a view... Now, let's take a closer look.
When you reach the top of the tower, look up and follow the prompt to scout the area.

Examine the points in the distance - this will add numerous new markers, mostly question marks, to your map across the Briar Sloughs area. Reward: 100XP.

4. Ending
Climbing that tower paid off. If I stumble upon other towers, I may use them the same way.