<p>When you sign in, they give you a big packet of stuff. One of things is a list of classes in each department that you can sit in on (as well as times and location). For those classes, you can just walk in, take a seat, and listen and/or take notes if it so interests you.</p>
<p>Now suppose you really want to sit in on a quantum mechanics class but you’re not sure if it is in the schedule, just send an e-mail to the professor (I think Broholm teaches quantum :)) and I’m sure they’ll be glad to let you sit in.</p>