Given Peter Eisentraut's blog post on the same topic, I thought I would also share this Zsh function (from 2011):

startup () {
    # info on any running screens
    if [[ -x $(which screen) ]]
    then
        ZSHRC_SCREENLIST=(${${(M)${(f)"$(screen -ls)"}:#(#s)?:space:##([0-9]##).*}/(#b)?:space:#([0-9]##).*/$match[1]})
        if [[ $#ZSHRC_SCREENLIST -ge 1 ]]
        then
            echo "There are $#ZSHRC_SCREENLIST screens running. $ZSHRC_SCREENLIST"
        fi
    fi
}