We formally verify the result checker for priority queues that is implemented in LEDA. We have developed a method, based on the notion of implementation, which links abstract specifications to concrete implementations. The method allows non-determinism in the abstract specifications that the concrete implementations have to fill in. We have formally verified that, if the checker has not reported an error up to a certain moment, then the structure it checks has behaved like a priority queue up to that moment. For the verification, we have used the first-order theorem prover Saturate.