It is a non-Retina, 2560-by-1440, IPS LCD backlit, with 75% less reflectivity than previous iMacs.
Interestingly enough, Apple claims:
Anyone able to confirm this, i.e., that new iMacs might not need calibration?
Make a grayramp/gradient in Photoshop similarly pictured on my LG above. Look for neutral color uniformity throughout from black to white with no or little banding. Go a step further and Posterize the gradient setting it to 21 and see if each step shows the same neutrality.
Make sure you use the default profile created by the Apple OS which I'm sure they've built accurate enough matrices for displaying color managed previews by loading some test images you already know what they look like on previous calibrated displays.
I'ld trust Apple's factory calibration claims because my $330 LG 27ea63v-p came factory calibrated out of the box and the gradient was perfectly uniform but a bit on the green side and after measuring with the Xrite CM found their 6500K claim was off by 200K.
I adjusted the RGB gains just slightly to make it a bit warmer which delivered an XriteCM profile of 6500K as seen loading the profile in Photoshop's CustomRGB dialog box in Color Settings. The grayramp went a tad bit red brown after the LUTs were loaded. Not that much of a change. Made my Dell's uncalibrated state look pathetic.