Take a look around the tree during the daytime to see if you see signs that they are using the tree regularly to roost in (droppings, feathers). Also, you could stay back from the tree a long way and use binos to see if you see any birds fly up to roost there right before dark. If you have seen a few birds in the area, even if they aren't roosting in that tree regularly, there are probably more in the vicinity. Set up before daylight and listen for gobbles as day breaks and either try to call them in or set up in that pasture edge and try to catch them coming into that area later in the morning where they may be strutting/breeding, etc.