Alright. I'll be on in a bit, gonna eat some dinner right now.
Most monsters will start limping once they're weak enough to be captured. It's more difficult to see underwater, but if you watch how they shift their weight when they swim away then it should be easier to spot. Flying creatures will usually limp to a specific place in the area before flying off to their rest spot.
A much easier way to see it is to have the Capture Guru armor skill (Perception +10). When the monster is painted, their symbol on the map will begin flashing white once they're weak enough for capture.
The only difficult thing about that is you have to zoom the map in if you're playing with three other people, and it becomes difficult to see the painted symbol at all when all four of you are right on the monster.