1. Qualifications of Author
Limit your articles to scholarly/peer reviewed articles and you'll generally be able to find the qualifications of the authors on the first page of the article. The qualifications generally consist of advanced degrees and affiliations with colleges or universities. If you're not sure how, go to Tips for Academic & Scholarly Articles.
2. Purpose of the Work
You can frequently find the purpose in the abstract of the article. If there is no abstract, the first section of the article generally states the purpose.
3. Main Points
This you will have to find throughout the article. Frequently there are different main points to each section of the article. It might help to jot them down as you read.
4. Whether it is Useful
This you will also find through reading the article. You should ask: Did I learn anything? Did the authors discover something new about the topic? Did they provide data or statistics? Did they do their own original research such as experiments, human studies or surveys or did they review existing articles--or both? The more you learned or discovered something new, the more useful the articles is.
5. How it Compares
You will have to read all of the articles for your annotated bibliography before you can determine this. Once you've read them all you can compare them to each other. You should ask: Which article did you learn the most from? Which article made the most important points? How many sources did the authors use when doing their research?