I have a CentOS 5.x server and I initially had it configured with a mount point. The netfs service is enabled on boot. /etc/fstab has an entry pointing to a cifs share://foobox.example.com/share /mnt/foo cifs  rw,credentials=/foo/cred,dir_mode=0775,file_mode=0664,softAt some point in time, the netfs service was disabled from starting on boot (runlevels set to 0).  An application on the server started writing files locally to /mnt/foo.So my questions are two fold: 1) How can I quickly check to confirm if a directory is a mount point elsewhere?  I was using df -h but I wasn't sure if there was a more convenient/helpful command. 2) What does linux do if a directory used as a mount point has pre-existing files?  Will I see both local and remote files?  Will it delete the locally-stored files? Will it hide them? 
How is locally-stored data handled when mounting/re-mounting a directory in CentOS?
centos;mount;cifs
1- You can use mount|grep <directory name> or simply mount command also.2- The preexisting files will not be accessible because the directory is pointing to the new data and not the previously existed directory. In other words, No change will be applied on previously existing files and you will not see the previous files, But they are existing on the disk and will not be deleted after mount.