Having a file open is not a lock because, if each process has to check whether the file is open first and not proceed if it is or create/open it if it isn't, then two processes could quite well check simultaneously, both find that it isn't open, then both create or open it.
To use a file as a lock, the check-and-lock operation has to be a single uninterruptable operation. You can achieve this in a Unix filesystem by creating a file with read-only mode and removing it to unlock. If the file exists (and is read only) the file creation will fail, so you get check-and-lock in a single atomic operation.
If your locking process is a shell script that will be running as a daemon, you can get this effect by using
umask, a per-process setting that sets the permissions that new files are created with:
umask 222 # create files unwritable to owner too
if echo $$ > /var/lock/foo
: locking succeeded
: locking failed
This also writes the owning process' PID into the file, which solves your other problem:
As regards the specific question "Which processes have this file open?", this can be useful when you want to unmount a filesystem but can't because some process has a file open in it. If you don't have those commands available, you can ask
ls -l /proc/*/cwd | grep '/var/lock/foo$'
or, as a mortal user:
ls -l /proc/*/cwd 2>/dev/null | grep '/var/lock/foo$'