Of course there are valid rentals on Craigslist, however these days it seems like the rental section on the site is swamped by IAC advertisements, its getting harder and harder to find actual private landlords. A good rule to follow is if there is a picture of a floor plan on the ad, chances are its IAC.
As far as scams go, if anything at all seems fishy, chances are its a scam. Here's an example, when I was renting out my condo, I received a reply for a little too eager tenant. Said he was out of country, was a medical doctor and would be moving in with his family. Would provide cash up front and could move in at the time I desired. Sounds like everyone's dream tenant. You ask, how could this possibly be a scam? Well, here's the catch, he would be sending me a down payment, 2 months rent plus security deposit. That would add up to almost $6,000. A good chunk of change for a new tenant moving in. HOWEVER, he would later state he would be sending me a check for $10,000, and that I would deposit it and write him a check for the difference.
That's the scam. I would deposit the check, and since banks take a little while to realize the check is bogus, most eager landlords would write a check to the scammer and by the time you realize the check bounced, you're already out $4,000.
Of course, its different when you're a tenant. I would request to meet in person, at the property, and I would definitely bring someone with you. Always meet during the day time, and never pay anything up front before viewing the property (an empty dwelling would be a good sign its legit), Request a copy of the lease to review, and if you do have to make a downpayment to hold the place, make sure to get a receipt. I would copy the person's license plate just to be sure, since you likely won't be asking for their driver's license.
On the other hand, working with an agent is a good idea too, since in CA landlords pay the referral fee, not the tenant. And chances are, its about 5-10% of the rent, so the landlord is paying a good sum of money to find a worthy tenant. So that may be the safe and secure way to go, but you may not always find the best deal, depending on the relationship with your agent.
Good luck and happy hunting..