How do you tell if the current terminal session is in GNU screen?
Check for the environment variable $STY (contains info about the screen) or for $TERM being 'screen' (probably less reliable).
signine is correct
See the doc
http://www.gnu.org/software/screen/manual/html_node/Environment.html
STYAlternate socket name. If
screenis invoked, and the environment variableSTYis set, then it creates only a window in the runningscreensession rather than starting a new session.
check $TERM, it is set to 'screen' in screen session.. (but not 100% guaranteed)
UPDATE
alternatively, you can utilize the fact that in screen, $TERMCAP contains screen substring:
[[ $TERMCAP =~ screen ]] && echo "in screen"also not 100% guaranteed
UPDATE2
if neither approach works, you can find some inspiration in screen manual