I'll take even the smallest evidence of success
I have made it my mission to institutionalize the value good API design. I strongly believe that this is key to making developers more productive and happy on our platform. One very small way to measure that success is to notice where 3rd parties have internalized that message, followed our guidance and use that fact as a selling point for why their product is better. Someone just forwarded me just such a site: https://www.extremeoptimization.com/Mathematics/Usability.aspx... Here is a sample:
Consistency
Consistency both with Microsoft's Design Guidelines for Class Library Developers and internally, ensures that you can reuse what you have learnt building applications for the .NET framework.
Do you know anyone else doing the same thing?