Accuracy of an estimated design wave condition such as the 100-year significant wave height is important information for risk assessment in design of marine structures. Assessment of accuracy of design conditions is already not simple when they have been derived from in-situ wave measurements at the project site. When using wave hindcast data this task is even less straightforward due to often a lack of sufficiently reliable measurement data near the project site for comparison. Different hindcasts may give significant discrepancies in prediction of extremes. Also, the quality of the hindcast may vary over time. In practice, we often do not much more than illustrating the quality of the data, for example by scatter plots of hindcast data versus measurements far away from the project site. In order to assess the quality of a design wave height from a hindcast systematically, errors in both the local hindcast data and in the extrapolation from these data need to be addressed. The paper discusses the overall idea and some building blocks for such an approach, while realising that there will not be one simple recipe applicable in all situations. One of the issues is the selection and use of observational data in quality control. Satellite wave data are an attractive data source for this purpose. For illustration, data from the seas around Norway are used.