Scouting in Sparta is the simplest form of attack. There is only one thing that affects the success or failure of a Scout mission.
The Number of Scouts on both sides
Note: Agreement upgrades have no effect on scouting missions or a Scouts ability to scout.
In most cases it comes down to a simple matter of whoever has the most Scouts wins. You should also note that in a Scout mission the defender NEVER loses units no matter how successful the attacker is. A success simply allows the attacker to see what is on the target city. Win or lose, if there are Scouts out on the base the attacker will almost always lose Scouts. That is the cost of doing business, and raiders should factor that into how they spend their resources.
Mechanics work much the same as normal combat except both sides use the Scout stat.
Attacker Loss Percentage = Defenders Scout / Total Scout.
Conversely you can calculate the Scout strength of a target in a failed Scout by this formula:
Defenders Scout = (Attackers Scout / (1 - Attacker Loss Percentage)) - Attackers Scout
Note: This only works if you have troops come home alive. Don't expect to use this if you sent a single unit and it died.