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I've heard many things about the author of "Ritva on Gittin." It might have been the Ritva, it might not have been. There's also the "חידושים מכתב יד," which some ascribe to the Ritva, while others don't. (see, eg, this answer)
How have people accurately determined the authorship of these works?
This is discussed at length in the hakdamah to the Mosad R' Kook Ritva, and in R' Avrohom Shoshana's Ritva published by Ofeq Institute.
In brief, the old Ritva acc. to some is ר' כרשכש. The new one acc. to some is Ritva, acc. to R' Shoshanah it's mostly the Ramah with some others. It's more complicated than that, but that's the Cliff Notes answer.
