How can you tell ?
I mean, do you have a 55 inch 4K next to a 55 inch 1080P?
I have seen two high quality large TV sets, 4K and 1080P in a store next to each other, and the there was no Night and day difference. Unless I got very close. At normal viewing angles/distance, the difference was minor for me. Money better spend on OLED.
Now take a monitor, which is in your face, 4K comes into its own.
What do you mean how can I tell? That's a pretty silly question.
You are assuming because YOU can't tell the difference, no one else can and basing your argument on the fact that YOU can't see the difference. MY Eyes can clearly see the difference. I don't want a monitor in my face. Did you say you need an 80 inch TV to see the difference? Now you are saying wow, on a 4K Computer monitor it's so much better? Are you trolling?
Just because yours CAN'T doesn't mean it's not there. And I have an OLED and an LCD. 4K Content even makes 1080p Content look superior because 4K TVs will upscale 1080p.