The best thing Forbes could do would be to set up an economic tracker for California.
Importantly, only a limited number of businesses are required to notify the state before they leave, so are easy to track. Most businesses just shut down, pack up and leave, with little or no notice, as long as they pay their final taxes before going.
But along with business departures, they could track state unemployment, food stamps, declining tax revenue as well as increasing tax rates across the board, durable goods orders, and many other business metrics.
The most important part would be to associate economic decline with particular policies. This would be very instructional to other states as to what *not* to do, and projected impact if they make similar law.
