Find out what Fiefs to Take
Go to Main Menu and choose <H>erald. From Herald Menu, choose <A>ctive players. Choose enemy nobles through <F>ind option. Look through fief list that follows.
If you are French, you want to go after English held fiefs in France. If fiefs begin with "F" they are in France (likely Gascony & Guyenne, here) and are fair targets.
An English player has an easier time of it. Usually, the English strategy is to go after the French kings fiefs. Otherwise, the English will target a particularly active French nobles fiefs, who may be distracted if all of his fiefs get taken.
A well organized king usually designates one player to keep track of enemy fiefs and post a regular report to the Council. The Kings Marshall or Connetable can then assign players with armies specific tasks. This teamwork approach works quite well and prevents unfortunate accidents in the field, where two armies from the same side go after the same fiefs.