Go into View menu --> View options (or right-click on the column headers in iTunes) and select "Bitrate"/"Data rate", and iTunes will show a Bitrate column that will tell you exactly how many Kbps a song is encoded at.
For individual songs, this information can also be found in the Information window --> File menu --> Information
If you wish to change the format and bitrate YOUR OWN ITUNES imports CDs at, you can do so in iTunes Preferences... --> Advanced --> Importing