The most credible sources are published by universities (i.e. Cambridge, Oxford, Princeton, etc.). Though there are few publishers that have a great reputation for history resources like Wiley-Blackwell or Routledge. You can find book reviews from history journals via databases like JSTOR and ProQuest. If you're registered at a college or university you will have access to these at your school's library (or you can see what your public library offers).

Be very wary of the history section at your generic bookstore. Most of those publications pander to the general public who crave sensationalist stories and don't know any better. And forget about wikipedia altogether.

Edit: This is an AskHistorians mod approved booklist that will get you well on your way.

Pretty much do what you have been doing.

Certainly reading the works of other historians- or listening to a variety of podcasts will help you get a more full picture of a historical event, time period, or movement. If for instance you only listen to one person describe the causes of World War I and they focus only on imperial-colonial conflicts and see the war in Europe as an extension of that- well nothing they say in inherently wrong- but others may prefer a different approach that stresses nationalism as a cause, or industrialization, etc... So none of them are necessarily not-credible, but they will tell the story in very different ways, and then you can decide which you prefer, or how effective the arguments are.

If you have access to Jstor, or another academic database, looking up book reviews can be a great way to judge the general academic feeling towards a book; especially if you have look at several of them. Generally though, academic journals will mostly just review academic books rather than popular historical works.

I suppose to some extent you have to ask yourself how 'deep' do you want to go in history. If you are looking mostly for narrative, then you can probably check the truthiness of a work against works of a similar nature; such as reading about how heroic Napoleon is- read several accounts of him if you want to get a better idea if he is a hero or not, some biographers are more guilty of hero worship than other, and certainly writers coming from the perspective of people attacked by Napoleon- esp. Egyptian or Russian, will take a very different point of view. If you are looking for books that tackle theory- and supply not just narrative (or maybe not even narrative) but seek to understand historical structures- seeking to explain why something happened, not just how it happened, then academic works might be appealing. Depending on the subject area you could post here for recommendations or look online for iTunesU courses, or online syllabus that give you some idea what sort of works a history course might include.