Almost every snake you'll see in a San Gabriel Valley backyard is harmless. The big one people see most often is the gopher snake — it can grow to 5 feet, has a blotchy brown pattern, and will sometimes flatten its head and shake its tail in dry leaves to imitate a rattlesnake. It's bluffing. Gopher snakes are non-venomous and they actually do you a favor by eating rats, gophers, and mice that would otherwise chew through your garden hose or get into your garage.
The other common ones around here are the California kingsnake (black-and-white or brown-and-cream banded — they eat other snakes, including young rattlers), the racer, and tiny ring-necked snakes that you'll mostly find under flowerpots. None are dangerous to people.
The only venomous native snake in our area is the southern Pacific rattlesnake. They have a triangular head, a thick body relative to their length, and obviously a rattle. Here's the thing nobody tells you: rattlesnakes do not chase people. They want to be left alone. Almost every rattlesnake bite happens because someone tried to pick one up, kill it, or stepped on one they didn't see. If you see a rattlesnake, back away slowly and give it space. It will leave.
What we tell our friends: don't grab any snake you can't ID. Take a photo from a safe distance and look it up, or ask us — we're happy to help. Killing every snake you see is bad for the local ecosystem and usually unnecessary. The snakes are eating the things you don't want in your yard anyway.