Google Chrome
Click the 3 horizontal lines at the top right and select 'About Google Chrome' from the dropdown. A new tab will open with the version number around the center of the page (i.e. Version 39.0.2171.95 m)
Internet Explorer
Click the cog at the top right and select About Internet Explorer 11 (it should display the version number).
Mozilla Firefox
Click the 3 horizontal lines at the top right and select the small question mark down the bottom. From there you will be able to select About Firefox, which will display the version number.