"Critical appraisal is the process of assessing and interpreting evidence by systematically considering its validity, results and relevance" (Bandolier, 2007, para. 1).
Reference: Burls, A. (2009). What is critical appraisal? http://www.bandolier.org.uk/painres/download/whatis/What_is_critical_appraisal.pdf .
Critical appraisal uses intrinsic (research design etc.) not extrinsic (author, journal, institution) factors. You'll need to consider the following when appraising an article:
Reference: Bandolier. (2007). Critical appraisal. http://www.bandolier.org.uk/booth/glossary/Critapp.html
These videos will provide an introduction to the concept of critical appraisal.
There are a number of checklists / tools available on the Internet to assist you in undertaking critical appraisal of evidence. Some examples are listed below.