The following documents from the CONSORT (Consolidated Standards of Reporting Trials) website. You can use them to critically appraise published trials.
MOOSE (Meta-analysis of Observational Studies in Epidemiology) Guidelines.
The PRISMA website contains the recommended guidelines for appraising systematic reviews and meta-analyses. PRISMA stands for Preferred Reporting Items of Systematic Reviews and Meta-Analyses. The PRISMA Statement outlines the background to the guidelines.
Reporting levels of evidence is an important part of critical appraisal.
In Australia NHMRC Levels of Evidence are used. However institutions around the world may use other levels of evidence. Always cite the source of the level of evidence you are using. See the following PowerPoint for an overview of available levels of evidence:
It usually comes as a surprise to students to learn that some (the purists would say 99% of) published articles belong in the bin, and should certainly not be used to inform practice". Greenhalgh, p.31 2010.
How to Read a Paper: The Basics of Evidence-Based Medicine
Critical Appraisal of Epidemiological Studies and Clinical Trials by 
