After you have gathered your evidence, you need to critically evaluate it for validity, impact, and applicability.
In general—when you appraise the evidence you have gathered, you are evaluating its:
Validity: closeness to the truth
Impact: size of the effect
Applicability: usefulness in a clinical practice
You can further break down the appraisal process by asking specific questions depending on the types of study that you are gathering your evidence from:
Check out this great Appraisal tutorial by the Medical Center Library at Duke University and the Health Science Library at the University of North Carolina at Chapel Hill.
S. Straus, W.S. Richardson, P. Glasziou, R.B. Haynes. Evidence Based Medcine: How to Practice and Teach EBM. Third Edition,2008.
UAlberta "Evidence Based Medicine: Mini Manual"