No evidence is or probably will ever be available. There are innumerous ways to "influence" without leaving trail.
Maybe the public will never know the truth but go outside, look around you and feel the weather that is now surrounding us in Doha. Its only May. Imagine how it will feel in a month from now. How pleasant the weather will be to play football and to celebrate and party in the streets and beaches of Doha with a pint or two of beer!
Just imagine this scenario and you will realize that somewhere in the bidding process, something went terribly wrong!
God will punish severely the cheaters regardless who they were (if any).