I would test in the same way using the same processes and methodologies that I would test anything else. However as Cognos is an out of the box software that is configured for the company using it for the Cognos functions I would only expect to do black box testing. For the Data Cubes, Marts and Reports that would be available I would probably do more verification against the originating data.
I have not failed. I've just found 10,000 ways that won't work" --Thomas Edison