given that there isn’t any objective way to prove someone’s high and not just having consumed within a couple days

You can do blood tests for THC, which are a reliable measure.

You are probably thinking of THC-COOH urine or saliva tests, which indeed will show positive for days or even weeks after you are already sober again, since THC-COOH is a metabolite of THC that takes very long to break down and does not have psychoactive properties

Just do it how Canada does it, they have a very reasonable cut-off value for THC blood concentration that even regular consumers with a tolerance for THC won’t surpass while sober again