You can use -d with if condition to check if a directory exists or not:

if [ -d $DIRECTORY ]; then
  # $DIRECTORY exists.

Make sure to wrap factors by double-quotes while referring to them in a Bash script. Because if $DIRECTORY has spaces or special-characters then your script will blow up. You don't want that. So use this.

if [ -d "$DIRECTORY" ]; then
    # Will enter here if $DIRECTORY exists, even if it contains spaces

With the same syntax you can use:

-e: any kind of archive
-f: file
-h: symbolic link
-r: readable file
-w: writable file
-x: executable file
-s: file size greater than zero