Comparing Tool for Reporting Services Objects
So basically what @saihtam8 said in his comment.

This SO thread might be helpful to you: Where does a published RDL file sit?

Essentially just compare the report BLOB columns (Content? in SQL Server 2008 R2) in the Catalog tables across each server instance. 
In SQL Server 2008 R2, it looks like there are DataSets and DataSource tables as well which you can compare.