Question
STM32L4xx reference manuals System clock (SYSCLK) selection
RM0351 and RM0392 (and maybe others, haven't checked them all) contain this in the System clock (SYSCLK) selection section
Status bits in the Internal clock sources calibration register (RCC_ICSCR) indicate which clock(s) is (are) ready and which clock is currently used as a system clock.
However the various clock ready and system clock switch status bits are actually located in RCC->CR and RCC->CFGR.