How do you scout cities which is in the daily tasks menu?

Find any other players city in the world map, as long as they are not in the same alliance as you. Tap on it and select the scout option. This will send a helicopter to check their city for resources / troops etc. It requires one of your APC's so make sure you have at least one unused. The task requires that the cities you scout are at least level 6.

Note that scouting will warn the other player that they are being scouted and can be seen by most as a warning of an attack. I'd suggest you scout players at a lower level than you and not in a strong alliance otherwise you might get attacked yourself. You can message the player to let them know its just for the quest,... unless of course you do want to attack them.