I don't know what edition you have. But the one I have has Christopher's comments and then indents everything that was written by Tolkien.
In his will Tolkien names his son 'literary executor' over all of his unpublished works. Chris could edit, add, burn, whatever he felt like doing with his fathers unpublished works. And as he remarks in his Introduction to Unfinished Tales, that he wanted to bring forward his father's unpublished works to the public for those who were interested in reading more of what his father wrote about Middle-earth. Just as he was. Also several times Tolkien names CT as a helpful collaborator/critic to him:
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A map of the Gondor area is perhaps the most urgent. I am hoping to get my son Christopher to produce one from my drafts, as soon as possible.~Letter 150
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'This book {LOTR} has become to be more and more addressed to you, so that your opinion matters more than any one else's.'~Letter 78 (Dated Aug. 12, 1944 to Christopher)
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And eventhough if Chris was a helpful collaborator to his father, and also was handed over the powers of literary executor, to do what he wanted with any of his father's works that were unpublished. I guess to answer in a round about way, Chris remarks that he never wrote anything himself, he comments on it, but didn't want to add anything himself (though he very well could of).
I don't know if that's a big help to you...I hope so. The edition I have makes it rather easy as anything indented is from Tolkien, anything not is commentary from CT . (and the Notes at the end of each chapter are also comments from CT).