During requirements engineering, use the requirement elicitation technique of observation that was previously mentioned to observe how users go about their work. This includes the process they go through as well as the current software that they use (if that software is relevant to the one being built). Through this technique, you can glean what works for the users, and just as importantly, what does not work.
Other requirement elicitation techniques, such as interviews where you can ask questions, can provide information regarding usability as well. The results of requirements elicitation can be used to create requirements related to usability.