Run the program below -

Set the port to output

TOPP:
Toggle LED
PAUSE 1000
GOTO TOPP

If the LED stays on for one second and off for one second, then
you are running the frequency you have set in your DEFINE

For example:

IF have DEFINE OSC 4 -

and the above program turns the LED one second ON and one second OFF
Then you are running at 4MHz

If it is on for 200milliseconds at off for 200 milliseconds. Then you are running
at 20MHz.