Test firmware for outputting the clock signal to an I/O port that may be loaded with a standard 10X probe, is included in the .zip file distributed with this application note. The crystal electrodes should not be measured directly if you do not have very high impedance probes intended for such measurements.
Compile the source code and program the .hex file into the device.
Apply VCC within the operating range listed in the data sheet, connect the crystal between XTAL1/TOSC1 and XTAL2/TOSC2, and measure the clock signal on the output pin.
The output compare pin will differ from device to device, so you need to look into the code to find which I/O pin will output the clock signal.