Much of science involves measurement of objects or states of the world. Often instead of direct measurement, scientists employ the use of indicators, or surrogate measures, changes in which are assumed to reflect changes in the target state. This leads to the problem of validation – ensuring that the indicators are actually measuring what they are intended for. Validation is a particular concern in cases where the target state is ‘hidden’ and cannot itself be measured for comparison. In this paper, I outline an approach using the concept of robustness, that can help with validating indicators of hidden targets.