This is a
very important test and a good question to make sure the device you paid THOUSANDS of dollars for is actually doing what it says it's doing. Good thing is, it's easy to at least make sure you're getting the minimal performance.
First thing, get your room totally dark. Wait till night if you have to, you're testing screen uniformity and it's crucial to have no glare or things to throw you off. Set your screen to 50% brightness as this is where baseline calibration is done.
1,
load up this image, it's full black. Makes sure it's even across the board, this is the easiest test since it's the bare bones requirement any screen calibrator will look at.
2,
load up this image, it's 80% grey. Slightly brighter than before but here is the crucial part, make sure it's 100% even across the board. No darker spots, not areas where it looks "dirty." Usually low end screens have a problem with this test but hopefully your 3,000 dollar imac doesn't
3, l
oad up this image, it's 20% grey so fairly bright. Same exact thing as before, make sure it's not getting darker by the corners and it's perfectly smooth across the full screen.
4,
load up this image, it's 100% white. An lcd screen should have zero problem making this look perfect but we're throwing it in anyway.
If your screen is failing at even one of these and you're still under apple care/return window,
do so immediately. There is absolutely no reason for that poor of quality control on something like this. Slight variances in color temp and tone are unnoticeable to anyone who isn't a full time photographer but for basic users, screen uniformity is absolutely a problem and it'll only get worse over time.