Many of these rows of trees still cut across the urban landscape, even where the creeks are today buried in culverts. There are other clues you can look for. Traveling along many large suburban boulevards, you may be crossing creeks, or the flood control channels they have become, without ever noticing, for the crossings do not look much like bridges. But if you see short stretches of matching high-security fencing on opposite sides of the street, that's a clue. Such fencing is frequently erected to discourage junior hydrologists from chasing water striders in what remains of the creek flowing under the road.
Maps are a great help, of course.
Several types are useful. California State Automobile Association maps do a good job of showing remaining urban creeks in northern California, but sadly, the same is not true in southern California. The U.S. Geological Survey topographic maps are great, especially in less developed areas. Creeks are clearly marked, as are the ridgecrest divides between watersheds. But topo maps are often out of date because, typically, the urban landscape has been altered since the map was produced.