For the longest time, this is always one of the first things I do when I set up an account on a Unix box on which I will ever be running screen.
I add this to my .bashrc file:
# Change the prompt if screen is active
if [ "$WINDOW" != "" ]; then
PS1="S:$WINDOW \u@\h \W> "
PS1="\t \u@\h \W> "
That gives you a nice prompt like this:
S:9 dave@nexus dave>
when you are in screen, where the S:9 says that this is screen window #9. Man does this make life simpler some days.