Hey guys. Sorry, I've been really busy at work, plus I'm getting ready to move this weekend. I haven't had much time at all to check the forums. There are a lot of very helpful people around here who have been answering all kinds of questions, I see (2000+ posts since I last logged in...), so hopefully I haven't been missed

. I'll try and login a little more often. That being said, yes, you can find out what is on a nandroid backup without restoring it. It's *tad* bit tedious to set up, but once you do, it's really simple to find out.
So, there are probably a dozen ways of doing this. I'm sure you can do some stuff with mount, but I couldn't get it to work right, off the bat, so I'll play around with that later. Anyway, if you're using windows, like me, the easiest thing to do is to:
1) Download and install
cygwin if you don't already have it.
2) Download
unyaffs. If compiling isn't your thing, you can get it precompiled for windows,
here.
Install cygwin (default installation is fine), then drop unyaffs.exe in C:\cygwin\bin (assuming that's where you installed cygwin).
Now, copy whatever nandroid backup you like from your phone to your computer. Create an empty folder somewhere on your drive. I'll use C:\nandroid as an example. Open a command prompt, and type this:
cd C:\nandroid
C:\cygwin\bin\unyaffs C:\path\to\nandroid\system.img
Just replace the cygwin part with whatever install path you used (or add that path to PATH if you know how to do that - I won't go into details here) and replace the second part with the path to the nandroid backup you just copied over (the system.img file, in particular). That will "extract" (not exactly the right word) the contents of the system.img into C:\nandroid, where you will see exactly what you would in /system if you were to restore that particular nandroid backup.
Now, in that folder, you'll find a file called build.prop. That contains information about your current ROM. You can open it if you want to, or take advantage of the fact that you have cygwin by typing:
C:\cygwin\bin\grep description build.prop (again, it's easier if you've added the path to cygwin bin to PATH)
The output should look something like this:
# Do not try to parse ro.build.description or .fingerprint
# ro.build.description=3.29.651.5 CL252548 release-keys
# ro.build.description=3.29.651.5 CL252548 release-keys
ro.build.description=Fresh Evo 3.3.0.1 by flipz
As you can see, this was a pretty old backup. When I did the test, I had no idea what it was. I now know it is Fresh 3.3.0.1. For those who aren't used to linux/unix typical text files, a "#" mean the rest of the line is a comment. So, the other two lines with the ro.build.description entry are the default (If it wasn't a custom ROM) and have been commented out.
Try this with any system.img that is in a whole nandroid backup (whether you know what it is or not) and you will find out what it is. Setting up cygwin take a few minutes, but you only have to do that once. Once you've done that, you can check any system.img you like, and it only takes a few seconds for each. Just delete the contents of C:\nandroid after each one.
Also, for those paying attention, you can do this with a data.img, as well, to extract apks (apps) from nandroid backups. Ever forget to make a backup of an app before flashing a new ROM? Well, here you go.
This doesn't work for boot.img files, by the way, so you won't be able to find out what kernel you have by doing this. I know there is a way to do it, so I'll go look it up later.