Leaked information is often correct. Today, those that leak the information use sophisticated technology to hack e-mail and other personal documents that were written by the person. Colin Powell has admitted the e-mails are his, and the information is accurate. It is rare that information that is leaked today is not accurate.
While the reader should always consider the intended recipient and what the goal was of the writer to their audience, generally a leaked document will be more truthful than not. The writer did not intend the content for mass consumption and was more than likely more honest and blunt than they otherwise would have been.
Ultimately I'd say I don't think anyone can be trusted, but leaked information is often valid, since people don't think it's going to be aired, and therefore may be a bit more candid. I believe Powell thinks Hillary is a screw up, Trump is a disaster and that Cheney is an idiot.
Information leaked from government sources should not be trusted under any circumstance and should be treated with a grain of salt. Without contesting the authenticity and motives of the leaker no information should be trusted. Any information that comes from a suspect government source should be viewed with a jaundiced eye and people should do more research to make up their minds.