Or, you could write a short delay routine, say for 0.5 second, and time two events, such as the blink of an LED. Here's a link to a handy calculator for delays:
Of course, that won't give the precision of an oscilloscope to calibrate the PIC internal oscillator, but it will tell you whether it is operating at 8 MHz or something way different. You can easily time to 0.1 S, so a 10-second delay would allow 1% precision.
Edit: Do you have a frequency generator?
John