There are some ways.
Log in as Admin
Open “settings” menu at bottom left and click “admin”
Scroll down to the bottom and here you go.
Check your status window of desktop clients
Open browser and open URL:
- Open your installation directory (e.g. using ftp)
- Check file “version.php”
- Search for a line like this
$OC_VersionString = 'x.y.z';
- x.y.z is your ownCloud version