What do you mean you don’t know how it would work?
People would check temperature with a thermometer at home before going in the train to work or sending little Johnny to school.
If your temperature is high (37.8 I believe) it can be a symptom of Covid 19 so you would isolate as needed.
The vast majority would follow that rule at that wold keep infection down.
You know like with lockdown how not everyone has complied but the majority have and by doing so the good people have protected our nhs and it’s not been over loaded. The good people have out weighed the arseholes