To find your version number, you will need to run the ME2 Launcher.
To reach the ME2 Launcher with the Steam version installed, find the location on your hard-drive where Steam is installed, then
Steamapps->Common->Mass Effect 2->Binaries. This is the same location you will need to go to in order to import your save from ME1.
For Non-Steam installs, the launcher should be the Start Menu shortcut created during installation, or it is accessible at the location you installed the game.
Once you have the launcher running (either Steam or non-Steam), the Technical FAQ says to select Configure and choose the Game tab. The number next to
MassEffect2.exe is your patch level.
Each installed Game in your Steam library has an individual setting for whether or not it should be kept up to date. You can verify that this option is selected for Mass Effect by right-clicking on the entry in your Library, then choosing Properties. On the Updates tab there is a check box entitled "Always keep this game up to date".