How to find alliance centers of other alliances in other states in ROC?

There is no easy way to find them. You can look for them manually or you could ask for help if you have any allegiances with players / alliances in the other state.

When looking for them manually though consider the most likely locations. AC1's are usually in the corners and always in the outer areas.

Most alliances place AC2 in the second band / zone of the map as close to the border with zone 3 as possible. AC3 is likely to be close to the border with zone 4 and AC4 is often near the center of the map.