How can I find my ally's army?

[Kírby]

Since I have a hunch that my ally has entered the war but isn't sending me soldiers because he's fighting himself, I'd like to find his armies-

Is there a possibility?

[Theandal]

click on your ally. He is most likely at his capital or leading his army. If he is leading army, your screen will move to his army location.
If you end up in his capital. You can look at his ruler page and in the bottom where Family is. There are tabs for relationships, courtiers, vassals. Find his marshal or a Knight. Click on them and game window should move to where they are.

[VipreRX]

Alternatively just look for armies with blue labels on the map. Considering most will be grey, orange, red, or yours they're generally easy to spot.
Narrow the area by seeing who he's at war with on his character page.