English Translation
I thought you were raised in L.A.
What is the か doing here?
The “か” here is being used to express uncertainty or supposition. It’s roughly equivalent to “I wondered if…” in English. In other words, the speaker wasn’t sure whether or not the person they’re talking about was raised in Los Angeles.
This use of か is usually found in combination with the verb 思う (to think), as is the case in your sentence. When combined like this, it denotes a past uncertainty that the speaker had.
@cstuartroe
As @Anxos clearly explained, particle か means “I wonder if”. Let me explain it in another way. The original sentence can be rephrased:
私はあなたがロサンゼルス育ちではないか？と思いました。