There are some signs that help to differentiate between the professional developers and amateurs. In fact, being a good coder doesn't necessary mean being a great developer for a startup, especially if founders of a startup are not tech savvy people. The development camp should also have good communication skills and understand the business side of the project. The capability of developers to perform the technical part of development can be determined from reviewing their past projects. The rest, can be evaluated by asking the developers how the development process is generally organized, how their activities can be tracked and what responsibilities they can handle. While discussing all these, the project owner will also see if everything is fine with the communication and mutual understanding. Another good way is to ask for references and ask founders of some previously developed projects about their experience with this development team.