Wonder at what distance your device is retina? Just run this app!
If the retina distance is less than the typical viewing distance then your display is retina.
You can calculate retina distance and PPI for any device.

Notes:
- Retina is the distance at which the pixels are so small that they are no longer individually visible.
- All displays are retina given enough distance. Therefore what really matters is actual viewing distance.
- The calculation uses a value of 60 pixels per degree (PPD) for defining retina.
Sometimes a lower PPD is used for retina calculation as low as 57.5 PPD.
- Most people will perceive displays as retina at a smaller distance than the one calculated by the app because the 60 PPD definition assumes perfect vision.

*Retina is an Apple Inc. trademark for high resolution displays