Butter is best before. Honestly can't remember which date type eggs have - we've entirely ignored them for decades. Never yet encountered an off egg, but they do thicken texture a little if you use the months old box you find at the back of the cupboard. It was an experiment, k? :)

Use by we respect - but even then only ish. Mainly as there is a lot of slack built in that date to allow for people with a too warm fridge, or it sitting in their nice warm car for 2 hrs first.

Generally we just go the traditional route to gauge: Smell, texture, separating, colour change, etc. Milk that's turned is super easy to detect. Was even easier before homogenisation.