You can find the Debian version on which your Ubuntu version is based in the file: /etc/debian_version
From 10.04 up to 17.04:
17.10 artful stretch / sid
17.04 zesty stretch / sid
16.10 yakkety stretch / sid
16.04 xenial stretch / sid
15.10 wily jessie / sid
15.04 vivid jessie / sid
14.10 utopic jessie / sid
14.04 trusty jessie / sid
13.10 saucy wheezy / sid
13.04 raring wheezy / sid
12.10 quantal wheezy / sid
12.04 precise wheezy / sid
11.10 oneiric wheezy / sid
11.04 natty squeeze / sid
10.10 maverick squeeze / sid
10.04 lucid squeeze / sid
sid is the development distribution of Debian (sid - testing - stable)
You can find out the contents of the file without installing an entire system by view the sources for the package basefiles on Launchpad.