I had only previously heard [0] that similarities in the tools were discovered by Kaspersky, not that there were any leaked docs that pointed the finger back at NSA themselves. Are you maybe thinking of PRISM/Wikileaks?
[0] - https://arstechnica.com/information-technology/2015/02/how-o...
And there are most likely a lot of cases where:
1) "...we got more than that," and...
2) ...data from "IPs and tools that are easily faked" is the only information that could be released publicly without compromising sources and methods.
It's a hopeless wish to want to be able to independently assess (as an amateur!) intelligence findings in all cases. If trusting the official assessments isn't acceptable (cross-checked with general knowledge of the situation), about the only reasonable alternative position is to remain agnostic.
Tools to fake such attribution and evidence were literally part of the leaked NSA/Equation Group toolkit.