Any typical scope probe should work for this. It the scope is showing a wave form but no change in amplitude, may need to change the voltage settings on the scope to provide a taller waveform to see a small difference in amplitude. Don't know what you have tried, but I would set the volts/division switch and the variable control to almost fill the screen with the waveform. Since you are only looking for a peak change in amplitude the volts/div doesn't have to be calibrated.