Well, if you set the default to NULL, it will return "", the empty string if the section isn't found. "" is rarely a good section name and you can check for that on the return.
Or, you could set the default to something you'll recognize, like "ThisIsTheDefault" and check for that coming back.
Other than testing for specific values, I can't think of a way to detect it since the API goes out of its way to have a default to hide the "missing" case from you.
Alternatively, you could use the other form of the API to get the enumeration of all the sections and explicitly look for the one you care about and detect that it is missing when you don't find it in the enumeration.
http://msdn.microsoft.com/en-us/library/windows/desktop/ms724353(v=vs.85).aspx[
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