How you can know "beyond a shadow of a doubt" that Treasurys are in a bubble

What are the classic signs of an asset bubble? People piling into an asset class to such an extent that it becomes unprofitable to do so.
Treasury bonds are so overbought that they are now producing negative real yields (yield minus inflation):

That’s right, after taking into account inflation, many investors in treasuries are standing over a drain and pouring their money down it. 
And so America’s creditors are now getting slapped quite heavily in the mouth by the Fed’s easy money inflationist policies.  (more)