We study the application of sums of squares decompositions and semidefinite programming in the formulation of computational criteria for distinguishing entangled from separable quantum states. A hierarchy of tests is obtained, the simplest of which corresponds to the well-known positive partial transpose (PPT) sufficient criterion, with the more complicated tests being strictly stronger. The duality structure of the problem allows us to provide an explicit construction of the entanglement witnesses corresponding to our tests.