I don't think there is an easy way unfortunately. Maybe you could make every object listen for mouse events and then somewhere note which one your cursor is currently over.
But honestly, if this is just for opening a menu I would simply run through all feasible Node2Ds at the time of the mouse click and then check which one Wait. Why don't you just make all your clickable items listen for mouse events and check if they have been right-clicked on? The one Node2D that gets this event was the one the user clicked on.