Well, one way is to buy lots of batteries. The old Tadiran lithium cells for computers (Macintosh 6100) all went flat about three years from purchase date, and from the manufacturer's data I estimated the clock drain at 36 uA. With some sheets of brass, I shunted the current through a VOM, and sure enough, it read... 36 uA.
So, within a few percent, you could drain a fresh battery to calibrate, then after a month or year of hive instrument operation, yank the used battery and drain it, noting the mAh difference.
The battery chemistry really IS a good, reliable indicator of current drawn No little spikes or slow leaks will get past the chemical change record.