I don't about testing API using the Silk Testing Tool.
But there are tools for testing API's of Application like Compuware Softwares BoundsChecker. You give your appliation, it tests the entire application and any API failures will reported. With the BoundsChecker you can check the Memory Leakes, OLE etc., You just evaluate the tool and know the more features. It comes with the Integration of True Timer, True Coverate.
Usually API is referred as a collection of functions or methods that can be called from different development tools.
So testing of DLL will be similar to testing any application except that tester should write a code to call those API functions from within development tool and then verify their behavior.
Also, lot of attention should be paid to documentation and usability. How easy for developer to declare and execute API functions or methods.
Silk Test does allow you to call functions in public DLLs however those DLL in most cases intended to be used in development software like Visual Studio. So I think it is makes more sense to test them using Visual Studio or macros editor in Microsoft Office.
[This message has been edited by ademin (edited 05-19-2001).]