1. Look for cottonwoods, willows, and sycamores. These are water-loving trees found in canyons where there is water in the soil. It may be that you hike to a grove of cottonwoods and there is a spring or pool of water or, in a dry year, you may have to dig down a few feet along the roots and see if it fills up with water. These trees can be seen from miles away and are known by their intensely bright green foliage (compared with the dull green of a pine or juniper tree). I have also relied upon aspen trees in the Great Basin Desert for locating springs. Palm trees in the Mojave Desert can also be harbingers of water. These trees are often the sites of oases in Africa and the Middle East.
2. North-facing canyons can often hold large pools of water in the sandstone or basalt basins. Reading a topo map can help with locating these, along with scoping such canyons out from a distance with binoculars (a must-have item for desert hikers).
3. Pay attention to insect and bird life when hiking. Once, in the Mojave Desert, after hiking miles through barren countryside, we came across butterflies and bees coming and going from a narrow side-canyon. We walked up the canyon a hundred yards and found a small pool of water under a protruding ledge. Being mindful of your surroundings can be of great benefit in finding agua.