Questions to ask yourself:
Based on what you already know about the subject does the resource seem accurate?
Do you find a lot of spelling errors or typos in the text? If a web site are there a large number of broken links or formatting problems?
Are there footnotes or endnotes or other documentation?
Does it contain a bibliography of resources used in the creation of the book, article, or web site?
If there is a bibliography are the majority of the sources academic or scholarly?
A book or article containing a large number of misspellings or typos or poor grammar may be written by someone without good credentials. At the very least there was not an editor involved before the final product was released - often an indication that the resource is not scholarly or academic work and certainly an indication that there was no peer review involved in the process.
Web sites with the same problems of typos, misspellings, and bad grammar are also suspect for the same reasons. In addition broken links often indicate that the person or persons responsible have not updated web site and the information on it. Bad formatting on a web site might include code that appears in the text, varying font sizes that don't indicate headings or subheadings or other breaks, or graphics that don't load, just to name a few. In other words, if it looks like a mess you'll probably want to stay away from it.
You'll find that most resources are reliable when the author(s) care enough to document their sources through footnotes/endnotes and bibliographies/list of resources. A quick scan of the titles involved will also tell you a great deal - a book citing The Journal of Information Science instead of Time or Newsweek as a source would be a good example of this.