The problem with 'how can you be sure' is that it is usually an attempt to cut off discussion and investigation, by implying that only irrefutable facts are admissible.
I'm not trying to cut off discussion. I was asking a genuine question because while it's entirely plausible the Russians did attempt to influence American politics a number of other scenarios are also entirely plausible. Are we just taking what the intelligence community say at face value now without seeing actual proof? That seems far more dangerous than whatever you're worried about. It's not like the US intelligence community have a flawless record when it comes to this type of activity either.
If the claims is really something as serious as "Russians overturned the result of the election" it should come with very clear evidence. Otherwise it is conspiracy theory. Granted it is a good one and a great PR campaign. But I think it has run its course by and it's time to switch to something else.
I also heard about "17 intelligence agencies agreeing on the fact". Maybe 25th Air Force (one of the agencies) can tell us more about what they have found there.
You seem a bit stuck on this thing about there being 17 agencies.
It was a DNI statement on behalf of the 17 agencies of the intelligence community. That means none objected to it. For the purposes of that statement the most important were the NSA, the CIA and the FBI. The FBI took longer to sign off than the others because they weren't initially convinced about the part that says the aim was to get Trump elected. They didn't have any problems with the part about Russian meddling in the election though. Apparently they got some additional Intel because they signed in the end.
This is way those three agencies have this logo on the statement, along with the DNI, but the coastguard didn't.
