How to spot if your website is built in ExpressionEngine 1, 2, 3, 4 or 5
Usually the version is noted somewhere towards the bottom of your screen in the control panel.
If you can't log into your website's CMS control panel, you may try to locate it by going to "yoursite.com/admin.php"
A dead giveaway you're site is in need of upgrading can also be the copyright notice on the login page: if it states Ellislab... you'll best update