"I use several methods to gather domain expertise. I typically start with meeting anyone within the company who is a current subject matter expert. I ask for a high-level overview and then set the stage for follow-ups as needed. I review any available documentation and gain access to any relevant systems to review them.
From there, I usually try to shadow end-users in the roles the product I am responsible for will serve. If that is not an option, I analyze the available market and demographic information. That helps me understand the user perspective so that I can design solutions that will best fit their needs and working style.
Finally, I would use whatever external information sources are available about the product we want to build or the technology we want to use. That might include testing competitor products, taking a class, reading journals, etcetera."