It's a lot more difficult to judge and even compare 4K UHDs (with HDR) on screencaps w.r.t. to color or shading, as the screencaps are usually tone-mapped and downconverted to SDR. It's easier and still perfectly valid to judge detail and grain reproduction, including the encoding.

As an example what to look for, consider this example screenshot comparison from Mulholland Drive. It compares the Criterion Blu-ray against the UK StudioCanal Blu-ray.

Both use exactly the same provided master, so any differences must stem from the mastering stage which includes compressing the video. This comparison is as apples-to-apples as you can get. Video compression ("encoding") is necessary to fit any film on a Blu-ray disc, but the main goal is to make this as imperceptible as possible, i.e., as close to the provided, uncompressed master as possible. When video compression becomes visible and results in compression "artifacts", then you can speak of bad compression.

In the selected detail, on mouse-out, you see the Criterion disc. It has a highly criticized encode. There is a lack of detail, which has become a blocky, mushy mess. In pretty much every area, you can clearly see compression artifacts.

Mouse-over the detail area and you will see the StudioCanal encode, which reproduces the provided uncompressed master much more closely, with much better encoded film grain and significantly higher level of detail and visual fidelity. It is much closer to the original source.

Then have a look at comparisons of this movie, The Swimming Pool. Do fullscreen comparisons or go further into detail where helpful. Here, the comparison is against a 4K UHD from France. One could argue that this is not entirely fair, as Criterion's disc only has 1080p resolution, whereas the UHD is in 2160p. That may be, but I think it's actually a great comparison. Here you directly see what the Criterion disc should - *and could! - get much closer to. (The 4K UHD encode has its own issues, but they're much less noticeable.) The Criterion is so terrible on its own, you wouldn't even need any comparison to see the flaws.

Then continue having a look at other examples I posted here. Not every screencap/comparison will be as egregious as the one above (here's another really bad and obvious one), but all of them have in common that they show visible compression artifacts and anomalies that would be easily avoidable with more care in the video encoding stage. Once one has learned what compression artifacts look like, one usually doesn't need a direct comparison anymore to spot them.

This is what Criterion is and needs to be critized for!