The linked site hosts a simple test. It shows two images with narrow vertical/horizontal lines. The more they resemble each other, the more Retina your display is.

Rails Long Term Support

Rails LTS provides security patches for old versions of Ruby on Rails (2.3, 3.2, 4.2 and 5.2)

Prevents you from data breaches and liability risks
Upgrade at your own pace
Works with modern Rubies

How to test whether your device has a true Retina display

Rails Long Term Support