It's very easy to mount the partition from a GNU/Linux O/S and view what's on it and how much free space you've got. If you haven't moved over to GNU/Linux yet then you could also check it from a live CD such as
Ubuntu Home Page | Ubuntu Just insert the CD re-boot and choose Live session from the menu. It will then boot into Ubuntu and wont touch your hard drive. Just plug in your phone and mount, the partitions should automatically open in a file browser after a few seconds.