a var_dump of the session shows the state is in a session named:
[WASF_CheckoutWizard_NewFromBlank_Default']['State']
so the code for the trigger rule should be:
if (true && (($this->GetTaxableTotal() > 0) && ((isset($_SESSION['WASF_CheckoutWizard_NewFromBlank_Default']['State'])?$_SESSION['WASF_CheckoutWizard_NewFromBlank_Default']['State']:"") == "CA"))) {