To be fair, there's been a few times where I'm just absolutely stunned as to how many troops the enemy suddenly has sometimes (I'm an experienced player, I check for alliances, special troops from prestiage/piety, etc.) It would be nice to be able to click on a troop and see exactly what its source is, I suppose. Even after all this time I still find a few things I can learn. That will probably never change, with a game this complex.
I had something strange going on in my current game. When I hovered over my opponent's portrait, it said that he had about 3100 men--800 and some from his demesne, and 2200 and some from vassals. He had no allies or tributaries or the live, yet his army on the map consistently had about 3800 men. Since I had about 3300, the difference in the size of his army between 3100 and 3800 was pretty significant. I still was able to win the war, but I certainly wonder where those extra 700 men came from. To be clear, I was checking his portrait during the war, not just before it, and it never showed him having any mercenaries or event troops or the like.