Stereo EQ Calibration method/sale advice needed

I'm going to sell my GML 8200 EQ, which I bought from a Nashville producer and I've used  both as a stereo and mono eq. Even though I didn't hear any unusual L/R differences when using it as a stereo EQ, I want to check how evenly calibrated both sides are and make sure it's within a sell able range as to avoid any hassles in the sale. So, as I'm inexperienced, I'm wondering what method people typically use. I do have Waves Q-Clone if that's useful. I also could just send it in to GML to have them calibrate it.

Any advice from anybody who has sold stereo EQ would be greatly appreciated!