Pick 2-3 features you actually use and show the improvement: "Constructor promotion cut our DTO boilerplate by 60%. Readonly properties eliminated an entire class of setter-related bugs."
Key PHP 8.x features: named arguments, match expression, constructor promotion, union/intersection types, readonly properties, enums, fibers, first-class callables. Strong candidates describe a real refactoring: reducing boilerplate with constructor promotion, replacing switch statements with match, using readonly properties for value objects, or using enums for state machines.
Tests whether candidates keep up with the language. Those still writing PHP 7.x style code are not leveraging modern tooling. Ask for a specific file or class they refactored.