When just emerging from the buds, the young leaves (petioles and some of the undersides) have a rusty tomemtum near the leaf blade attachment.
Fall color can be spectacular. Often less that so here. Usually more yellow that orangish yellow in this picture.
Bark is grayish brown in small shallow irregular shaped scales when old, smooth and gray when young.
Misidentification: Other buckeyes, at least around here you will find A. x carnea more often than A. hippocastanum. A. carnea has dry buds, not sticky, red flowers, smaller leaves and shorter more rounded habit.
Aptos - Cabrillo College, by the Sesnon house down the side of the hill towards the creek. Only one I know of.