Hi IronTek,
If you don't have an external clock, you can run in Self Clock Mode. However its real frequency is not known, an indication is given in the datasheet in the electrical characteristics.
Hi Bee,
The external clock is the absolute timing reference of the microcontroller. By itself only, the MCU is not able to determine at which frequency it is running.
The only way to determine the clocking speed is to have another reference like a PWM (or another regular known signal) watched by one of the TIMERs. By the number of counts in the timer, you will be able to calculate the bus/oscillator speed.
Cheers,
Alban.