Simple, look at the log.... The first few lines after a WeeWX startup tell you exactly what python version is being used by WeeWX, eg:
Jun 3 11:15:55 weewx weewx[2712] INFO __main__: Initializing weewx version 4.1.1 Jun 3 11:15:55 weewx weewx[2712] INFO __main__: Using Python 3.8.2 (default, Apr 27 2020, 15:53:34) #012[GCC 9.3.0] Jun 3 11:15:55 weewx weewx[2712] INFO __main__: Platform Linux-5.4.0-33-generic-x86_64-with-glibc2.29 Gary -- You received this message because you are subscribed to the Google Groups "weewx-user" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/weewx-user/14dfc253-ef8d-4fdc-9e5c-5b501de6e9e6%40googlegroups.com.
