How do you check which specific player is controlling the stated actor? Search and wiki didn't help.

Use a for loop to cycle through ActivityMan:GetActivity().PlayerCount. With each loop, check if self.Controller.Player is equal to the player being checked.
Don't forget that the player number starts at 0. That means that if there are 2 players, the players are 1 and 0. In your for loop, make sure to only loop the player count minus one.