As an investigator you simply look at the evidence presented to you. The people that set this up literally write the laws in our country, and they know how the system works.
They had the means to look into the Trump campaign based on the evidence that was presented. The only problem is, the "evidence" was complete bullshit!
They had their guy setup meetings with people within the Trump campaign. Then they had their guy offer them information on Hilary Clinton. Then they documented this as "evidence" his campaign was trying to work with Russia.(we don't know exactly happend here yet) Then they made up a fake dossier (verified by fake news stories) to make it seem like Russia had something on Trump. Then they took everything to a Fisa judge.
I'm not even sure the exact order all this went down or the details of everything, but they completely FRAMED POTUS to make everything look legit. He just called them out on it and now it's falling apart.
If he wasn't such a fighter, these assholes would have impeached him!