I just had hoped there'd be a way (apart from guessing or code-crawling) to tell if an XPCOM-IDL describes a service or not...
Docs are the best way. You can sometimes infer by the name of the interface (if the interface name has the word Manager, or Service). However this can be completely misleading in some cases.
If you find an IDL that you can tell if it should be treated as a service or not, and if is public, you should file a bug.
Doug Turner
