Firstly: you need a Castle to be able to scout other cities.
from the patch notes of 16-04-2010
- if all the attacking scouts die, no information is gained.
- if any scouts survive, the amount and type of all defending troops are visible
- if the number of surviving attacking scouts (SAS) is higher than the number of surviving defending scouts (SDS) the attacker will know the amount of goods that can be plundered.
- if the number of SAS is twice that of the number of SDS the amount and level of fortification is revealed
- if SAS is four times the SDS the amount and level of all buildings is revealed.
In short, to defend against scouting you need to have many scouts yourself in your city defending. Having lookout towers will help in this scout-battle since they double the effect of your defending scouts.
(SAS) = Surviving Attacking Scouts
(SDS) = Surviving Defending Scouts