Hello: No secret code. However, the link to the APP for a screen test is in the iTunes store. As of August 23rd, 2017 you have Version 4.0.
https://itunes.apple.com/us/app/touchscreen-test/id1126113478?mt=8
You can test the pixels, touch, pinch, move, multi-touch on 6s and beyond.
Remember, aftermarket replacement screens DO NOT provide the moisture seals, response, and drop performance so please use Apple Care or a Certified replacement brand such as eVero. Your iPhone is worth it.
Enjoy, have fun.