There are multiple things to look for in backend developers, including:
Solid grasp of specialize programming language like PHP, C#, Java, Python etc depending on your preferred technology stack.
Expertise in Web Frameworks based on the selected programming language. For example, Laravel, Yii are among top used PHP frameworks
Database knowledge in MS SQL, MySQL, MongoDB, PostgreSQL
API Development
Version controlling and Deployment skillset
Excellent communication skills
Proactive approach to testing their own work