Typical Ways of Measuring Usability
Learnability: pick novice users of system, measure time to perform certain tasks. Distinguish between no/some general computer experience.
Efficiency: decide definition of expertise, get sample expert users (difficult), measure time to perform typical tasks.
Memorability: get sample casual users (away from system for certain time), measure time to perform typical tasks.
Errors: count minor and catastrophic errors made by users while performing some specified task.
Satisfaction: ask users' subjective opinion (questionnaire, interview), after trying system for real task.