Pretty simple, Send a Scouting party first (if you wanna know how much resource he / she has)
If you decide he/she is worth attacking for the resource, and your scout report fails to yield the Defense units the castle has.
Send in a Common (disposable) hero along with 1 scout and attack the target.
You will surely see how many units he/she has defending and the most you loose is time and 1 scout