If you will interview a C# developer you will find this checklist useful since it will help you asking more advanced questions in your interview with C Sharp developer.
He should already tried to learn more than one non-related programming language to C Sharp. This will show that he can learn always.
I believe it's a big plus if he already worked with any open source databases like MariaDB, MySQL, PostgreSQL. Also, he should be familiar with Git as a source version control tool.
Utilizing open-source packages always a good sign that this developer knows that open source packages always are mature enough because it is widely used by other developers. I'm always waiting to hear about these packages (Entity Framework, Dapper, AutoMapper ...).
He should talk about API endpoints that he built before. And the authorization using JWT. It would perfect if he is familiar with Swagger and HTTP status codes.
Getting to know the syntax of any template engine and know any of these two popular options is a must.
He should explain in general the principles and popular architects that he have worked with like Clean architecture from Microsoft, Union architect, Domain-Driven-Design, MVC, N-tier, Repository, microservices. Also, you can ask if he start building the project Code-first or DB-first and why he go with this option.
He should mention any public standards not only internal standards in his team. Sometimes I'm looking to hear about SOLID principles. Clean code.