The problem is you can't prove a negative. You can have strong suspicions or beliefs, and lots of evidence, but you can never be conclusive - at least not the standard expected by the usual suspects.
But Boardman was clearly both a supreme technical innovator and a pretty talented cyclist. More than that it's hard to say. His willingness to have samples for posterity is also helpful. As, frankly, is his relative candour about seeking a TUE, because frankly, you hear testo, you think dope.
The current British revolution will have to be written up one day. One way or another, however it turns out. And Boardman, Obree and Queelly will deserve their chapters as much as Wiggins, Hoy or Froome, or Millar and Tom Simpson.