It depends. Most of the time you can just hover over the little Moodle icon at the bottom of the frontpage (first page you see). MOST of the time it will tell you the current version.
However, I have just recently run into an interesting situation where I upgraded from 1.5 to 1.7. Unfortunately, when I hover over the Moodle icon, it still tells me I'm running 1.5.
About the only way to know for sure which is running is to go into the Moodle files themselves and look for one called version.php. When you open it, it will tell you the current version you are running.
As a teacher, you won't be able to do this. Only the admin can.