It sounds like you found a house that someone is advertising for rent. If the house has already been foreclosed, then that means the bank owns it. If the bank owns it, then it's possible they're trying to rent it. Not likely - but it's possible - stranger things have happened.
So, ask the person who is trying to rent it to you for more details. If the bank owns it, then they're going to give it to a real estate agent to rent for them. If such is the case, then simply ask the "landlord" if they are an agent and if they say they are, get their license number and then check that with the DRE (Department of Real Estate) which you can find online through a search engine such as google.
If they say they're not, you can delve deeper by asking additional questions such as "are you the owner"? If they say they are, get their name and check it against county property records as the first step. If that checks out, ask them to show you a copy of their grant deed. Ask for a copy of their identification (driver's license). While this kind of thing is unusual to do for most people, it will keep you from getting scammed. Don't be surprised if they show reluctance, but express your concerns and see what they say. If they won't prove it, walk away. If they tell you they're out of town - run.
If they tell you they're the property manager - again - ask if they're an RE agent and then do as mentioned above. To manage someone else's property in CA you have to be an agent so don't be shy about making demands.
You can also knock on neighbor's doors and ask them if they know the person trying to rent it to you and/or what the status is. They often know what's going on.
Hope this info helps:)
If you would like help finding out if this property is a rental and not a scam, I would be happy to help you. You can either call or email me through Trulia. Representation is always helpful in buying or renting homes.