What is an appropriate transition rule for checking that the player has landed on the ground and performed the landing state to transition back into an idle state?
well that depends, if youre using a CHARACTER blueprint then you will ahve a character movement component. (for this example ill assume youre using the Third Person Character Blueprint)
so in your animation blueprint, in the update event you can do TryGet Pawn Owner > Cast to Third Person Character > (drag out the blue pin) Get Character Movement Component > isFalling
promote isFalling to a boolean variable and that can be used to transition back to yo idle state.
if youre NOT using a character blueprint and using a pawn or actor, you will not have a character movmement component, in that case, its up to you to figure out how to check if youre pawn is still in the air or not in the Pawn blueprint, and then move that over to the animation blueprint.
Hope that answers your question.