Ask your prospective game developer to explain their process to you. If they have to formulate one on the spot, move on. Creating modern games is a highly complex endeavor. As with professional software development of any type, there must be a process which the developer follows from project conception to post development, if the project can be expected to go smoothly.

While a few different strategies exist for managing software development projects, Agile is, by far, the leading system of methods and practices used by professional Unity 3D game developers.