[BorisVSchmid]

It basically comes down to data availability, and some historian digging through the available archives.

For example, Givry is a tiny place somewhere in France, but for which by happenstance the parish registers at the time of the Black Death survived. They record the daily number of deaths, which end abruptly when presumably the priest died after recording ~620 deaths since the beginning of the epidemic in that town. The town's population is estimated at around 1170 people, based on the death rate in the earlier months, leading to a lower end estimate of 53% mortality (if the death of the priest was also the end of the epidemic).

    (from page 306 of http://www.persee.fr/doc/bec_0373-6237_1939_num_100_1_449196)

    [0 0 0 0 2 0 0 0 0 0 0 0 0 0 0 0 1 1 0 0 0 2 0 0 0 0 0 3 2 0 0] ;; jul. . approximate start of epidemic somewhere during jul.
    [0 4 4 1 0 2 1 7 5 3 1 1 4 5 3 3 4 1 5 2 2 4 3 6 2 2 2 6 3 8 4 6 10] ;; aug.
    [6 9 8 8 4 6 15 3 11 24 7 10 15 10 14 11 17 8 6 3 17 6 11 16 7 16 12 5 10 7] ;; sept
    [7 6 7 7 8 7 6 6 9 4 7 9 3 14 5 5 4 8 6 7 3 2 5 4 4 2 3 1 3 0 6] ;; oct
    [3 4 3 4 0 2 1 0 4 3 2 1 0 0 5 0 0 0 3] ;; nov, records end with the dead of the priest, worst of the epidemic is over.