request.get_host()
https://docs.djangoproject.com/en/dev/ref/request-response/#django.http.HttpRequest.get_host
but be aware this can be cheated so don't relay your security on it.
If users are seeing your machine under same address I am not sure if this is possible via runserver (it is supposed to be simple development tool).
Maybe you could use nginx? Or if this is only for testing do something like:
for i in 1 2 3 4 5; do manage.py runserver 10.0.0.$i:5000; done
and then sys.args[2] is your address