Next to the icons in the party list you can notice some white lines building. Those are the enmity meters for each one of your party members. I use that as a guide when I tank and swap between sword and shield as a PLD. If you rotate on targets it changes. As a tank however if you feel like your losing control you can always mark a target for assistance until you re gain control. Just keep in mind when switching over to shield oath it is recommended that you are over geared for that location. If not you may be making the job harder for your healer.