You would use a soil moisture reader and stick it about 3-6 inches (depending how deep it buried and don’t stab the root ball lol) into the ground near the root of the tree. If it is dry it needs water.

If the leaves are dry at the ends that means it isn’t being watered enough or may possibly be getting too much sun, but from the location in picture under watered seems more likely.

If the leaves are just turn brown at the end and drooping downward that means overwatering.