Add spies, you can send them to another country to get some information like:
Army size, How much gold they have, How much gold they earn, How much they can recruit etc.
There is a chance the spy will get caught. But, the weaker the nation, the lower the chance he gets caught. If he got caught the country might decrease relations with you.
If a country sends a spy to your country, they will get your info and see if you're weak so they can attack you. If your country is strong, they might fail and you get a message that [country] sent a spy to you, then you can decide what to do: