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.
