Below you can find few ways on how to determine CoreOS version number.

Method 1

First method is dome simply by login. Every time you login to your CoreOS system a "Message of the day" located in /etc/motd is displayed:
Last login: Thu Dec 10 09:05:41 2015 from 10.0.0.8
CoreOS stable (835.9.0)
Alternatively, see the content /etc/motd:
# cat /etc/motd 
CoreOS stable (835.9.0)

Method 2

Another and preferred method is to consult the content of /etc/os-release file:
# cat /etc/os-release
NAME=CoreOS
ID=coreos
VERSION=835.9.0
VERSION_ID=835.9.0
BUILD_ID=
PRETTY_NAME="CoreOS 835.9.0"
ANSI_COLOR="1;32"
HOME_URL="https://coreos.com/"
BUG_REPORT_URL="https://github.com/coreos/bugs/issues"

Method 3

Both methods above only gave us CoreOS version number and version ID. To retrieve a CoreOS code along with a version number see a content of the /etc/lsb-release file:
# cat /etc/lsb-release
DISTRIB_ID=CoreOS
DISTRIB_RELEASE=835.9.0
DISTRIB_CODENAME="Red Dog"
DISTRIB_DESCRIPTION="CoreOS 835.9.0"
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