When evaluating a websites or other sources, it is essential to go beyond the original source and see what others are saying about it.
Think about it... you probably wouldn't trust a chef's review of his own food, because he has a vested interest in promoting his restaurant. Reviews from objective restaurant-goers would be much more credible, since they have no personal stake in the restaurant's success.
The same idea applies to evaluating websites. Approach content written by authors about their own websites skeptically. Authors are likely to promote their brand or services, rather than share an honest review of their work. To get an accurate idea of credibility, you must seek out information from other sources.
This technique is called "reading laterally," because it involves moving sideways from the original source to gather information from a variety of other sources.