Well, as I posted to you last night - this op could not have been done without the highest approval. Having said that, you are right that it probably cannot be legally proven - and perhaps should not be for the sake of the country; there are many high on the chain of command that will have to fall on their swords to save POTUS #44 if the issue manages to advance that far.
Being even more specific, _Vachel speculates that 50 USC 1802 was used - this requires a signed approval by the AG, and this represents the president's approval ...
Notwithstanding any other law, the President, through the Attorney General, may authorize electronic surveillance without a court order under this subchapter to acquire foreign intelligence information for periods of up to one year if the Attorney General certifies in writing under oath ...I don;t believe this statute was used to spy on the Trump campaign. Like Bush's "TSP" before, the snooping was just done by identifying targets, with no resort to anything resembling judicial process as 50 USC 1802 does.
I believe Obama at least knew and allowed snooping on the Trump campaign (was kept in the loop, possibly directed it be undertaken), and if testimony from those around him could be elicited, that testimony would prove it. But I don't think I could identify any hinky paperwork that I think might implicate him. That is my "I don't think I could prove it."
As you point out, too many high level people are aware of and participating in complementary schemes to believe Obama was unaware.
Judge Napolitano, on Fox 3/14/2017: "“Three intelligence sources have informed Fox News that President Obama went outside the chain of command. He didn’t use the NSA. He didn’t use the CIA. He didn’t use the FBI, and he didn’t use Department of Justice. He used GCHQ."— TessaBD (@TessaBD1) May 26, 2018