Hallo Jose,

if you use an external (well known) watchdog and some Flash data memory you can try it. Have a loop running which increments loop counter and updates entry of certain Flash data memory cell, by storing updated loop counter data. E.g. '1', '2', '3', ...
Keep the watchdog unreseted. When the watchdog then resets read the content of flash memory cell, and you can estimate clock rate.
I have not spend much time to anaylze whether this method really works. But when watchdog reset time is much bigger than loop delay time and if this loop delay time is much bigger than (rather undefined) flash writing time, a certain estimation of clock rate should be possible. At least, to decide whether F/6 or F/12 mode is running. Or, am I wrong?

Bye,
Kai