Phases can also be inferred by using a process called molecular replacement, where a similar molecules already-known phases are grafted onto the intensities of the molecule at hand, which are observationally determined.  These phases can be obtained experimentally from a homologous molecule or if the phases are known for the same molecule but in a different crystal, by simulating the molecules packing in the crystal and obtaining theoretical phases.  Generally, these techniques are less desirable since they can severely bias the solution of the structure.  They are useful, however, for ligand binding studies, or between molecules with small differences and relatively rigid structures (for example derivatizing a small molecule).