I have a 15" rMBP (Mid 2012) running Mavericks. In addition to he built-in Retina display, I mirror to an external Dell U2410 Display/Monitor connected via a Monoprice "Min DisplayPort + UB to Dual-Link DVI Adapter." To my old eyes, I see no difference in image quality between the two screens, yet the internal monitor is set for "Best Retina." How can I determine what resolution I am getting on each screen? I'm still new to Apple, so I will appreciate detail in your reply. Thank You.