The link below provides guidelines on how to critically evaluate a scientific article:
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2696241/pdf/Dtsch_Arztebl_Int-106-0100.pdf
The following links provide checklists for the critical appraisal of systematic reviews:
http://apps.who.int/rhl/Critical%20appraisal%20of%20systematic%20reviews.pdf