It's pretty easy. Just get a breathalyser, that's what I do in countries with "zero" (below detection level) BAC tolerance laws whenever I've been drinking previously. The alternative is getting breathalysed by the cops, which means automatically losing your driving licence if positive. I add half an hour for good measure, the cheap models aren't that precise.
Or at the very least use a BAC calculator app – very roughly, one light beer (UK pint) at lunch could take around 2‑3.5 hours to eliminate, depending on your sex, metabolism and weight and other circumstances. Use a calculator with your own weight and add half an hour for good measure.
It's not really that hard...