In my universe there are more than just one FTL method: Wormholes, Warp, Hyperspace and more. The thing is I need to predict the destination of an enemy fleet.
I only can think about spies and hacked systems, or maybe some sensors on asteroids which send a signal to the empire so they know something is coming.
Scanning or something like that would not work because of the size of the universe and the size of the attacking empire.
It is a very far away future and they have a lot of technologies which they could use, but are there more ways to track the enemy down?