If you do not know the process ID, you will have to check all processes that have the same fd# open, since file descriptors are not globally unique. The smaller the fd#, the more processes will have it open (e.g. on my system, even if the fd# is around 30, I'd still need to guess between 15 processes, and if I was looking for fd# around 10, then the list would have ~170 processes).
The proc filesystem shows the file descriptors as symlinks under /proc/<pid>/fd.
# ls -l /proc/1/fd lrwx------ 1 root root 64 Feb 12 22:10 /proc/1/fd/0 -> /dev/null lrwx------ 1 root root 64 Feb 12 22:10 /proc/1/fd/1 -> /dev/null lrwx------ 1 root root 64 Feb 12 22:10 /proc/1/fd/2 -> /dev/null l-wx------ 1 root root 64 Feb 12 22:10 /proc/1/fd/3 -> /dev/kmsg lrwx------ 1 root root 64 Feb 12 22:10 /proc/1/fd/4 -> anon_inode:[eventpoll] lrwx------ 1 root root 64 Feb 12 22:10 /proc/1/fd/5 -> anon_inode:[signalfd] lr-x------ 1 root root 64 Feb 12 22:10 /proc/1/fd/6 -> /sys/fs/cgroup/systemd/ ...etc...
For example, to look for fd #5 in all processes:
# ls -l /proc/*/fd/5 lrwx------ 1 root root 64 Feb 12 22:10 /proc/1/fd/5 -> anon_inode:[signalfd] lrwx------ 1 root root 64 Feb 12 22:15 /proc/129/fd/5 -> socket:[6980] lrwx------ 1 root root 64 Feb 12 22:15 /proc/168/fd/5 -> socket:[7847] lrwx------ 1 root root 64 Feb 12 22:15 /proc/341/fd/5 -> anon_inode:[eventfd] lr-x------ 1 root root 64 Feb 12 22:15 /proc/342/fd/5 -> anon_inode:inotify ...etc...
The exact interface to resolve symlink targets is readlink():
# readlink /proc/427529/fd/7 /home/grawity/lib/dotfiles/vim/backup/%home%grawity%.bashrc.swp