I agree that there isn't a way to know if a header should be added implicitly, until Generator is called.

A possible change to the Generator behavior is to generate the header in the serialized output, without modifying the original.  I don't know that it makes debugging easier, but it would keep the Generator from having this side effect.

Maybe being explicit is better, a utility factory could be used to keep the common case simple.  I don't know if such a change is reasonable at this point.