How do I find out what the CLSID is going to be for my DLL when I register it?

The DLL project name is "eth0" and the DLL class name is "CShellExt". I compiled and registered it, searched the registry for "eth0.CShellExt" and found it under the key "{5BFAC3A1-8B25-44E0-8A38-0FA0980A5355}" so I made the following .reg file
Code:
REGEDIT4

[HKEY_CLASSES_ROOT\jpegfile\shellex\ContextMenuHandlers\eth0]
@="{5BFAC3A1-8B25-44E0-8A38-0FA0980A5355}"

[HKEY_CLASSES_ROOT\giffile\shellex\ContextMenuHandlers\eth0]
@="{5BFAC3A1-8B25-44E0-8A38-0FA0980A5355}"

[HKEY_CLASSES_ROOT\pngfile\shellex\ContextMenuHandlers\eth0]
@="{5BFAC3A1-8B25-44E0-8A38-0FA0980A5355}"
But it does not seem to be working. How would I check that I am using the correct CLSID?