They use a machine where you look through the lense and they can split an image into 2 and then back to 1. Based on how long you see 2 images vs 1 they are able to tell if you have convergence insufficiency.

I know if you read online the definition of convergence, it says you see double images but that wasn't the case for me. I had difficulty reading for more than a couple minutes or spending more than 30 min on a computer. I did vestibular therapy and doing the tracking exercises would always make my symptoms horrible but since I had no problem actually seeing things in focus, didn't think vision was the cause.