Except for a lightweight trace, there is no way to know that in SQL Server 2000 (not sure about SQL 2005). Oracle does have a nice feature to flag the indexes (kind of like a toggle switch) that get used - it doesn't have anything to show how many times it was used.
But during benchmarking of a product, a lightweight trace and running all the scenarios that the user can potentially run-through in the application seems to be a nice way to get this done.