In short: you need to send more scouting power (Flying Dutchmen and Zeppelins) than they have defending them (FDs, Zepps, Crow's nests).

Of course, this won't tell you their fire power, only the strength of the troops currently in the stronghold. They could have much more in their harbour, not to mention the reinforcements that their BH and other friends might send them when they see that you've launched a raid.

The intention here is not to discourage you, but do plan carefully and - if you decide to hit - do hit quickly, to reduce the chance of being surprised and ambushed.

Good luck.