What are their sources? Check their footnotes, see where the information comes from. If it is from a secondary source, find it and check their footnotes. Ideally, you should find a primary source. That isn't the end of it though. You also want to evaluate how that source is being used, as sources need to be approached critically. Say there is dispute on battle casualties, as you say. We check the sources, and in the end, it turns out that one writer is using the reports from the country on their own losses, while the other is using information surmised from archaeological excavation. Then we want to look at their methodology... did the first accept those reports uncritically? What methods did the latter use to extrapolate casualties from the findings of the excavation? Or, perhaps, they end up using the same source, but interpret it differently. In that case, look into how they came to their respective conclusions. For a source which is imprecise like that, ideally they will explain a little how they arrived at the number they did.