But a killer one is their past work. I've lost count of where they expect the newer developer to deliver on time. Everyone has to get a start and experience so if you can give them some slack that can result in them working hard for you and a long relationship.
If a developer has no demos or examples of their prior works, that's a bad sign. Even the newest developer has created apps to show off their skills.
What are the factors one needs to consider before choosing a developer for getting an app developed?