Differentiating between compressed/uncompressed dmg's

umm does anyone know a fast way of doing this?, my 60 gig drive is filling up and I was wondering if I could compress some dmg's to save space, but I don't know how to check which ones are already compressed

Yes you can compress dmg images. Most compression programs will place a new extension on the newly compressed file.

Will compression hurt the file at all? If not, why don't applications install themselves as compressed?

Compression should not hurt the file. If it is an application, you will not be able to use it until you uncompress it.

I was thinking more along the lines of using disk utility to convert a normal one to a compressed one, then there's no need for any extra software to extract it, panther just mounts it. My problem was telling the difference between a compressed one and a non-compressed one (both with extension .dmg) as I have quite a few cd game images and I can't remember when I started compressing them.