this largely depends on your future plans, such as if you plan to return upon your return to the US. my non-professional recommendation to you is to minimally get a lease w/ witness while signing it in new state, new drivers' license, and utility bills in your name with non-trivial activity. obviously this is not free, but consider it insurance and vastly cheaper than 11% + penalty backtaxes on CA income.
obviously you are welcome to skimp out on any one of these, but you'll have trouble convincing auditors you're a resident without it. note that NYC tax authorities have solicited location data for purposes of 183 day rule, etc.; i am not an expert in CA but precedent is there.
obviously you are welcome to skimp out on any one of these, but you'll have trouble convincing auditors you're a resident without it. note that NYC tax authorities have solicited location data for purposes of 183 day rule, etc.; i am not an expert in CA but precedent is there.