I was just looking into my credit report online and noticed that they have a link to "dispute" with 2 of the 3 CRA's online.
I don't know if this would be useful for demanding verification though, and you wouldn't have the paper trail either.
But, in case you are wondering:
http://www.transunion.com/investigate
http://www.experian.com/rs/fi2.html
And just in case anyone is looking for mailing addresses for them, they are as follows:
Equifax
P.O. Box 740241
Atlanta, GA 30374-0241
Experian
PO Box 9595
Allen, TX 75013
Trans Union
2 Baldwin Place
P.O. Box 2000
Chester, PA 19022