According to The Intercept, a classified excerpt from page from the NSA's internal wiki shows that the NSA once verified that Russian journalist Anna Politkovskaya's email account had been targeted by Russian Federal Intelligence Services a year before her 2006 murder. On the surface, that sounds about as vague as the current statements we have about the DNC hacks, but how its listed in the wiki reveals a lot.
The information is classified as "Top Secret Signals Intelligence" -- a term that denotes the tracking of signals as they travel from place to place. In other words, the NSA knows Politkovskaya's email was hacked by Russian operatives because they were able to trace the hack back to Russian intelligence. It's still vague. The entry itself doesn't specifically say how this trace was accomplished or provide the evidence -- but the existence of the entry shows that the NSA is wholly capable of tracing such hacks back to their source.
It's not hard proof that Russia interfered in the US election, but it's certainly evidence that US intelligence agencies are capable of gathering such proof. Unfortunately, it's also an indicator that such evidence is typically classified -- and not something the company is likely to release if it risks showing its hand to foreign operators. Whatever evidence US intelligence officials may or may not have, the doubts of the President Elect will be either verified or strengthened soon. Donald Trump is scheduled to take the oath of office on January 20th.