Du is your friend.

Start Terminal, login as root (devel-su), go to root directory (cd /) and type:

This command will tell you how "full" is every directory. If you need to inspect deeply, change the "--max-depth" number (2, 3, 4...) or enter the "suspicious" directory and, from there, repeat the du command with depth 1.


Another way: find.
This command will find files bigger than 100 megabyte. But I prefer du because find can discover big files, but not a lot of small files occupying the same size.