We are developing a product which provides an API interface, so other developers can use some functionality of main product.
This is already implemented and documented.
But I'm not sure that this is very useful, and documentation is very clear for developers.
How we can find people to review this feature? What type of person should it be?
In some sense, we're looking for Product Owner for single feature/component. Is it possible?
It would be the best to find a programmer that has already designed some known framework for the language in question.
WHat your users think is irelevent I think, becuase you can't judge about framework by asking arbitrary programmer his toughts about it. His answer will depend upon his level of knowledge and personal methodologies while API is being designed for wider population (I am assuming this is your case). To finish this point, in my firm programmers that use VB.NET think that C# is lame language becuase of 'problems' like you can't write Button = "Text" in C# and let the compiler automatically find default property for you. You don't want such person judging your framework.
Even designers experienced in different languages may be of help because wider used APIs should be implemented with the help of well known programming patterns people are already familar with.