How do I tell if a class is WSS or MOSS specific?

How do I tell if a SharePoint class is specific to WSS or MOSS?

For example, can I use the SPAudit class in MOSS only?

I do it by looking at which DLL contains the class.

For example most SharePoint API functionality is located in the Microsoft.SharePoint.dll (which is WSS). Another example is the Microsoft.SharePoint.Publishing.dll which is MOSS only.

The SPAudit class works for WSS as well as MOSS, even though the user interface is available in MOSS only.

Like JD Said, have a look at the DLL. If your API requires Microsoft.Office.* then it is MOSS only.

Have a look here for detecting MOSS using code.