Property-based testing is a widely used testing technique in the context of classical programming. It comprises identifying a set of properties that collectively encompass the functionality of a program, followed by implementing a set of property-based tests that generate inputs from a specified domain as test cases. Initial proposals have been put forward to apply this technique to quantum programs. Property-based testing in a quantum computing context removes the burden from developers to identify edge cases for potential program failure, due to the automatic exploration of the input domain. The main obstacle in the application of property-based tests in this context is the complexity of the final verification step, where the output state is to be characterised. This requires the application of computationally expensive assertions. Classical shadows have been proposed as a means to efficiently estimate observables of a quantum state and provide a viable solution to this obstacle. In this paper, we discuss how classical shadows can be used for the efficient implementation of assertions, leading to a more efficient approach to the verification of postconditions in property-based tests.