How do I know the colony is truly broodless before an oxalic treatment — cluster check, tray debris, or the calendar?

Third year in Uppsala, four hives, and I want to get the winter oxalic right this time. Last year I did an Api-Bioxal dribble by the calendar, mid-November, and my spring wash was disappointing on two colonies. The consensus at the club was that they probably still had brood when I treated, so most of the mites were sitting under cappings where the acid couldn't reach them. So how do you actually know a colony is broodless, without tearing the cluster apart in the cold? Do you open on a mild day and split the cluster to look at the centre frames? Is there a temperature cue — first hard frost plus so many days? Or is it the 21-day calculation from the last egg? First frosts were this week here, nights just under zero, days around 6-8°C. And a second question: if I'm unsure, is it better to treat twice to be safe, or does that do more harm than good?