That seems really difficult (and high-complexity) to me, whether you focus on identifying human agents or whether you try to do something much more general. Evidently you think otherwise.

This is motivated stopping. You don't want to admit any evidence for theism so you declare the problem impossible instead of thinking about it for 10 seconds.

Here are some hints: If you were dropped into an alien planet or even an alien universe you would have no trouble identifying the most agenty things.

You'd need your definition to pick out what parts of the world are to count as agents that can be involved in game-like interactions,

Well there you go, agents are things that can be involved in game-like interactions.