I ask candidates to sketch an API (program, not REST, i.e. an outline of classes, functions and module structure) for a problem similar to those we solve, but smaller in scope.

I also ask about things like test coverage and programming paradigms they have an affinity for. Some things I look for that can be negative signals are:

* Propensity to choose classes and OO programming in a domain it isn't really suited for, or functional/procedural programming where OO might fit better

* Propensity to build for what might be rather than what is (e.g. always have an abstract class even if only one concrete class is necessary)

* Propensity to maximize test coverage (e.g. splitting the logic of functions into smaller ones just to generate test cases, even when those smaller functions are only called at one location) rather than to design for a solution