Thanks to @Ignacio Vazquez-Abrams and @Jeff Schaller (and other commenters below), I realized that $DISPLAY was the enviormental variable I needed. If you are in a display server the value held by $DISPLAY <s>will be 0</s> will be returned (as something). Hence, the following code (should?) works--


    if [[ -n $DISPLAY ]];
    then
        PROMPT = (etc)
    else 
        PROMPT = (etc2)
    fi


Thanks to all.