you can right click on the file system and select properties or from control station run "fs_dedupe -l"

If you right click on a file in Windows Explorer and go to properties you'll see the "size" and "size on disk" - this I believe tells you how compressible an individual file is.  You should also be able to do this on a directory level.  I think this only relates to compression and not "dedupe" though I haven't read anything one way or the other.  PST files are generally already compressed.  Some compressed files actually get bigger when they are compressed again.  If you have millions of tiny files (like I do) you'll use up a ton more space than the actual data stored in the files.  On the up side, I've had 1GB TIF images compress down below 100MB and the time to open them compressed or not compressed isn't noticeably different.

Dan