Great question. Fact is, any brand new vehicle purchased outside can only be pass the smog check and subsequently be registered only if it is California smog compliant. The vehicle's emissions label must indicate it is 50-State Emissions Legal (California). If the label indicates the vehicle is only 49-State Emissions Legal, the vehicle cannot be registered in California until it is considered used (7500 miles on the odometer).
If buying an out of state vehicle through a dealership simply consult the salesperson. They will know immediately whether you may register it in California, soon after purchase. If buying from a private party, instruct the seller to read the vehicle's under hood emissions label. It'll be located somewhere towards the front of the vehicle either on the upper radiator support or affixed underneath the hood.
