Don't watch. Listen. Overpower has a very distinctive "woogie-woogie-whammo" sound that is easy to distinguish from other SFX.
But any pull with 4+ targets would benefit from Goad. You could apply it right at the start. Trust your instincts and play proactively.
Here is a handy chart:
1. Are there 4 or more targets? If Y, go to 2. If N, hold Goad.
2. Is there a WAR? If Y, Goad the WAR. If N, go to 3.
3. Is there a MNK? If Y, Goad the MNK. If N, go to 4.
4. Is there a DRG? If Y, you can't Goad KO'd targets. If N, hold Goad.