How can I tell if my mini iPad has Retina display

Append the last three characters of its serial number to http://www.everymac.com/ultimate-mac-lookup/?search_keywords= and load the page.

(101102)

Try checking with System Status App.

https://itunes.apple.com/sg/app/system-status-activity-monitor/id401457165?mt=8

This is a hardware report of the iPad 4 using SystemStatus (tap to enlarge image)

Put your iPad under light and look closely. If you can see squares then it isn't retina. If you can see tiny unnoticeable squares, then it is retina.