The UK has introduced a set of rules to govern UK Ministry of Defence's (MoD's) use of simulation across its programmes. The rules – the Defence Training and Education Coherence (DTEC) Ruleset - are designed to introduce efficiencies to MoD policy and acquisition, predominantly through actively encouraging re-use of simulation artefacts and consistency in architectural approaches in which the MoD has invested. To promote re-use, the MoD DTEC approach has started production of a DTEC Catalogue. The DTEC Catalogue is intended to inform the UK Defence community (MoD, Industry and Academia) of software, data and other artefacts that are available for re-use. The UK Defence Simulation Centre (DSC) capability has been established by the UK MoD to maintain the DTEC Ruleset and, in addition to other functions, to provide a central MoD capability to test simulation artefacts for inclusion in the DTEC Catalogue and to provide technical support to assist in their re-use. This paper discusses the rationale for developing the DTEC Ruleset and Catalogue and the approach employed by the DSC, which is currently in an interim DSC (iDSC) capability, in testing simulation artefacts for re-use, where the future specific use is not known at the time of testing, hence: Testing the Untestable.