For junior positions, I tend to ask very simple questions. Reverse a string or given an array and an int count how many times that number occurs (or variants thereof). Something that requires a loop and some ifs. It opens the posibility of endless discussions about extensions, refactoring, runtime, edgecases - it gives a solid bases on which to judge their ability.

For senior positions, it's slightly more advanced. But if they make it through to second round, we have them bug hunt and refactor some piece of code.

Programming is a craft and I need to evaluate that craft.