This paper proposes a general technique for testing decomposition of component specifications based on rules for formal verification. Component specifications are expressed as pairs of two models: a context assumption and a component guarantee. Thereby they capture the contract-like nature between the component to be developed and the context in which it is supposed to work. The paper provides empirical evidence that A/G rules developed for formal methods are highly relevant as patterns for testing decomposition of component specifications. More explicitly, that testing the validity of decomposition based on A/G rules is valid, and moreover, that testing based on A/G rules is more efficient that a conventional approach for the same kind of test-quality.