My guess is that the very vast majority of these documents are real and should be taken very seriously.
Now, it does look like they may be from DCCC and not the Clinton Foundation - Guccifer 2.0 in this case being a stupid, bragging and amateurish hacker.
From a single data point? You don't.
You need to be able to gain the same information from other sources to corroborate it, and weighted based upon trust criteria. A known good trustworthy source in your web of trust outweighs neutral sources. A series of confirmations from other, disparate, seemingly-trustworthy sources that do not have a likely bias toward the information also count positively.
At the end of the day some part of it is subjective, yes, but you can gain confidence in the accuracy of a source with third-party corroboration.
But we should look to corroborate that information in addition to corroborate information from the Panama Papers, given the importance and implications of the information and the incentives for the sources to alter and selectively leak information.
Sure, you could make that argument based upon your own web of trust.
Crowdstrike and others that have given this Russian intelligence attribution are higher in my web of trust than an anonymous Russian-appearing hacktivist, largely because they are people are in my own social circle and personal network. Believing that attribution and having an understanding of the history of Russian tradecraft and its current political leadership, I'll verify before trusting in this case. I'll assume something in here is likely faked, forged, or set up to deceive in some way.
Not believing it is Russian intelligence but likely highly partisan, I will maintain my skepticism given the timing of this release and the fact that this hacktivist has only been anti-Clinton late in an election cycle.
Good information warfare doesn't forge everything. It weaves the lies in with the truth so it becomes indiscernible what is truth and what is not. The truth is what sells the deception.