Who's writing this, anyway? What are the author's credentials? When you're evaluating a source, considering the author is a good place to start. Thinking about this person's educational background, publishing history, and professional affiliation (where they work) can help you to figure out a couple of important things.
1. Can this person be considered an authority on this subject?
2. Does this person have any bias that might impact their work?
When was this source published? No matter what kind of source you're using - print, electronic or otherwise - knowing when the information was gathered and published is important. Some fields, such as science and medicine, are constantly developing, making older sources outdated. On the other hand, fields such as literature and history may require you to use primary sources which were written during a specific time period. When you're choosing and evaluating sources you should pay close attention to the requirements of your topic or project and whether or not the sources you're choosing satisfy those requirements.
Who is the intended audience for this source? Is this a book written for children or scholars? As a researcher it's your job to make sure that the sources you ultimately select for your bibliography are appropriate. Something that is either too simple or too advanced won't be useful to you.
Aside from the author, where did this source come from? Where (or by whom) a source is published can often be tied to both who its intended audience is and whether or not the author can be considered an authority. A book that is published by a university press can probably be safely considered scholarly. On the other hand, a book that is self-published rests solely on the qualifications of the author, without the history and reputation of an academic institution to back it up, and a book that is commercially published is probably not intended for a scholarly audience. This is also true of internet sources. A webpage that is affiliated with an academic or educational institution is always going to be a safer bet than a page with a more questionable background.
Is the information in your source presented in a logical, objective, and impartial way? Is the author dealing in fact, opinion, or propaganda? Is there enough evidence to support the author's position?
Also consider to what extent the author's ideas differ from those of other experts in the field. The more widely an author differs from their colleagues, the carefully and critically you should consider their work.
The following links offer tips on how to critically evaluate sources