Perhaps you could test them against a piano or even a tuning fork ( concert 'A' 440 Hz )

On the internet you can search for accurate tuning tones as well.

I tested my Spider 6 amp tuning feature against an internet tone and it came out quite accurate. My clipon digital tuner, on the other hand, didn't fair so well. I have a Korg metronome / tuner that gives out various tones and that was quite accurate as well.

Joe.