When you plug your iPad into your PC and run iTunes and select your iPad on the left side of the screen under Devices, you will see a picture at the bottom of the screen that will tell you how much space you have used and how much you have left. (Of course yours won't say Scott's iPad!)
If you only want to see the gross amount remaining and used and not broken down into the detail that iTunes does then you can do this from the iPad itself.
'Settings', 'General', 'About'
and you'll see listed a whole load of stuff including the total capacity of your iPad and the remaining capacity.