As far as I know there's no way to tell. Last I checked the available drug tests could only tell if you've used cannabis in the last 30 days, not if you used it on the day you were driving.. and even if you had that doesn't mean you were high at the time you drove.
Drug tests should really be replaced with performance tests.
It's none of their business if you're high. What matters is if you can drive safely.
Seems that you are right that there is no reliable way of testing for this (and the 5 nanograms might have been pulled out of someone’s a…ttic when the law was written).
Cannabis is way more restricted and controlled than tobacco or alcohol in most places. If you buy cigarettes or alcohol, cops won’t bother you. If you’re caught with cannabis, you are potentially facing criminal charges.