There were other threads mentioning this earlier, and even when you put 5/6 military sabotage agents in a country, it does not guarantee you complete army reveal (I don't believe there is an exact probability). If you are going to keep tab on 5 man coalitions, it is best to coordinate with others in your team to split the funding. Money isn't too big of a problem when you don't use spies, so I'm sure if everyone can pitch in, you would be able to reveal all their armies.
Someone also mentioned about not putting spies on provinces with buildings in order to keep focus on revealing armies instead of blowing this up. That should help you increase probability of success as well.