Monitoring systems for measuring rainfall, as well as flow rates and pollutant quantities conveyed by sewers and/or discharged by stormwater overflow devices, have become a common feature in many municipal sewer services, in part spurred by recent regulatory requirements. However, the state of measurement conditions in sewer facilities does not always ensure reliable results. For this reason, it is essential that measured values be carefully screened prior to their use, since many sources of disruption capable of skewing data can be encountered. The present article describes a method for validating dry-weather data a posteriori. This method relies upon flow rate forecasts, a combination of standard daily wastewater flow profiles and an estimation of infiltration flows. Measurement results are then compared with this forecast and an appropriate series of statistical tests are run to detect all major data anomalies. A number of diagnostic rules are then applied in order to derive an initial interpretation of these anomalies and, in particular, to identify the influence of rainfall events.