The way I do it is first check for any settings that don't add anything to image clarity. I always turn off Motion Blur, Depth of Field, Chromatic Aberration, Bloom, and Film Grain. Even Sharpen if there is such a setting, should be left at default, because raising it beyond that can add a sort of dotted fuzziness called video noise.
Some games even have heavy filters that cause blur and mute colors, or add sepia tones, such as The Evil Within's Glare Filter. Fortunately they offered a way to disable it in an update via a command. A lot of the above options lower FPS when on too.
If it's a game that I struggle to get 60 FPS with, I'll use 1080p, then beyond the above options being turned off, I start looking at things like Shadows to...