The client ${\mathsf {must}}$ preorder relates tests (clients) instead of processes (servers). The existing characterisation of this preorder is unsatisfactory for it relies on the notion of usable clients which, in turn, are defined using an existential quantification over the servers that ensure client satisfaction. In this paper we characterise the set of usable clients for finite-branching LTSs, and give a sound and complete decision procedure for it. We also provide a novel coinductive characterisation of the client preorder, which we use to argue that the preorder is decidable, thus positively answering the question opened in [3, 6].