You could always draw it out and do sin/cos/tan, but that's a little complex and you would need a protractor. You could estimate its height by comparing it to a building whose height you know.
You can also hold a sick and move your legs, or the arm that is holding the stick, until the top of the stick seems to touch the top of the tree, and the bottom of the stick seems to touch the bottom of the tree. From there, you would swing the stick at a 90 degree angle and mark the point on the ground that the top of the stick seems to touch. Height of tree = distance from that point to the base of the tree.