How do I test a retina display to see if it’s eligible for repair do to pixel anomaly problems?

There is no test as such, but Apple provides the following guidance: About LCD display pixel anomalies for Apple products released in 2010 and later - Apple Support.

The Apple Store or AASP will need to judge whether the pixel anomalies warrant a repair or not. The total number, relative proximity, and location of defective pixels are several factors that will bear upon that decision.