For Windows;
Press Windows button + R and press cmd then click "OK".
Write down "Ver" or "winver" and press enter
For Linux;
Open Terminal and write down "uname -a" and press enter
e.g.
Microsoft Windows [Version 6.1.7601]
e.g.
Linux localhost.localdomain 3.11.10-301.fc20.x86_64 #1 SMP Thu Dec 5 14:01:17 UTC 2013 x86_64 x86_64 x86_64 GNU/Linux