The "proof" is in the form of feedback from forum members who have measured the input signals to their STBs, as well as industry insiders and Bell insider knowledge regarding their satellite signals. See the FAQ on Formats below:
The latest information we have, is that Bell (Sat) is converting all of the channels to 720P to save bandwidth. Bell Fibe, Bell Aliant, as well as Telus Sat and Optik get (most/all of) their signals from Bell - some (local area channels) may be direct from the "broadcaster".
Since they are short on bandwidth, there is no reason to believe they switched back to 1080i for some channels that broadcast 1080i since that takes additional bandwidth. Of course the channels that are listed as broadcasting in 720P in the first part of the FAQ would not be changed by Bell to 1080i.
So, the only programming that isn't 720P is programming that is OD, or the occasional local channel that the providers listed above may
receive directly rather than through Bell Sat, and transmit as 1080i. Until someone disproves the above information gathered over the years, we will maintain that Bell non-OD is 720P.