[Air-L] Criteria for proving that online data (especially forum comments) are real?

Hi, Would anyone know what are the criteria concerning internet material's validity? Is it common that you as a researcher will be asked to prove that the material you have collected from the internet is real. Since a lot of internet data disappears everyday, mere url addresses are not enough. Even html files can be modified after saving webpages. I would appreciate if someone had time to answer. Maria