During an evaluation, the primary goal of the person peforming the evaluation is to OBSERVE the employee(s) peformance as compared to the expectation or job description. In areas where the employee meets or exceeds the expectation, this would be considered a strength. In areas where the employee falls short of the expectation, this would be considered a weakness.
Remember, evaluation is all about observation, observing the employee's peformance and/or behaviors relative to their job description. Often times this happens throughout the initial training period, and then throughout the course of time prior to their performance appraisal.