If you forget to use pv and want a nice progress bar for an already executing process, use `pv -d <pid>`. That'll display progress bars for every open file. Works even for things like installers and servers, where you wouldn't be able to use a pv pipeline anyway.
If you want to emulate this by hand, first get the fd number of the file of interest by `ls -l /proc/PID/fd/` and then `cat /proc/PID/fdinfo/NUMBER`. There is a line called "pos", which is the position in the file.