Public authorities are increasingly sharing sets of open data. These data are often preprocessed (e.g. smoothened, aggregated) to avoid to expose sensible data, while trying to preserve their reliability. We present two procedures for tackling the lack of methods for measuring the open data reliability. The first procedure is based on a comparison between open and closed data, and the second derives reliability estimates from the analysis of open data only. We evaluate these two procedures over data from the data.police.uk website and from the Hampshire Police Constabulary in the UK. With the first procedure we show that the open data reliability is high despite preprocessing, while with the second one we show how it is possible to achieve interesting results concerning the open data reliability estimation when analyzing open data alone.