I have heard from reliable sources that the only way to get a true California sycamore is to clone one from a very old one in its native habitat. They hybridize easily with the London Plane tree and all the stock from nurseries more than likely are hybrids.
Other Sycamores, especially if it happens to be from a nursery. Look for the bark on an old tree but that is not always reliable. Look at the leaves, they have much longer lobes than that of the London Plane tree.
the best place to see london plane and the California Sycamore side by side in a mature landscape is on Sea Cliff Drive E. at the corner of Sea Terrace Way. The condos have 3 sycamores and one london plane tree.