If you want to know who attacks your Allies without asking them about it each time, you can set up the incoming battle reports about the attacks on your Allies' Cities. You will be receiving the battle reports after all incoming attacks, spies and sieges of their Cities.
Such option is available for all players in the game. However, it can be activated only if both sides configure the incoming battle reports of the Allies. Such option can be found at the Diplomacy Reports settings. When you open the settings you will find two tabs: My reports and Allies' reports. At the Allies' reports tab you have to select friends who will be receiving the battle reports about your City. Your Allies need to do the same if you want to receive their battle reports. Please note that you will not be getting the Allies' reports until they select you at their Diplomacy Reports settings.
Now when the Allies have been selected, please go to "My reports" tab and mark the types of the reports you would like to receive. Do not forget to Save the changes when the selection is done. From now on you will be receiving the notifications about interactions with your Allies' Cities.