Credibility usually does not have a yes or no answer. More often it is a continuum, i.e. some sources are more credible than others.
In the academic world the most credible sources are scholarly journals (also referrred to as refereed, academic, or peer-reviewed journals). These are original research articles, written by experts, that are reviewed by other experts (a process called peer review) before they are published. These sources are relatively rare on the Internet, and often charge for access when found.
Sources that are not scholarly are considered popular sources. Some of their characteristics are:
Within this category, some sources are more credible than others. The more credible ones feature: