Broad knowledge of .net Frameworks for developers and deep knowledge of platforms for integrator.
If development focused do they demonstrate understanding of OOP concepts, ability to apply common development patterns in .net and explain their methodology to manage the dev life-cycle such as their flavor of agile. A warrior's arrow fired without fins to guide it is a dangerous thing.
Do they think about the .net world? Curious developers will always evolve and that's a good thing. Ask which two emerging .net frameworks in two years will be on the rise and which two will be on the way to the grave, then ask the dreaded why?
Ask them what platforms they have worked with, when, and if they had to work with that platform again what would they not look forward to doing?
Some providers will be specialists, some generalists, depends on your project's needs, both have their place. If they haven’t used Quartz.NET and are building you a scheduling application, not the end of the world but if they don’t at least know it exists along with similar frameworks, that might be a warning flag.
In summary, determine if their knowledge of frameworks or platforms is sufficient for your project.