[ ! -t 0 ] && xset b off

-t

    file (descriptor) is associated with a terminal device

    This test option may be used to check whether the stdin [ -t 0 ]
    or stdout [ -t 1 ] in a given script is a terminal.

function xruns {
    if [[ `pstree -As $$ | grep xinit | wc -l` == 1 ]]; then
        echo "You are in X."
    else
        echo "You are not in X."
    fi
}

#!/usr/bin/zsh

CURRENT_VT=`tty`

if [[ ${CURRENT_VT[6]} == "p" ]];        # or `${CURRENT_VT:5:1}` in bash
then
   # X stuff
else
   # non-X stuff
fi

if xhost >& /dev/null ; then echo "Display exists"
else echo "Display invalid" ; fi