I like to use terminal colors so it's red, as well as allowing controlling width, but defaulting to terminal width.

  $ cat ~/bin/br
  #!/bin/sh
  if [ "$1" == "" ] ; then
          COLS=`tput cols`
  elif [ "$1" == "--help" ] ; then
          echo "$0: Prints a red line across the screen. or $0 <##> for a specific width."
          echo "$0:   br ; grep -ir foo *  -- place a marker to see where results begin / end."
          echo "$0:   br 80 ; cat file     -- use to check for overly long lines."
          exit
  else
          COLS=$1
  fi
  LINE=""
  for (( x=0 ; x<$COLS ; x++ ));  do LINE="$LINE-" ; done
  echo -e '\E[47;0m'"\033[1m$LINE\033[0m"