My kids used to sit on on math classes every time we visited a campus. Math departments generally don’t mind. They would check the online schedule, choose a class, say hello to the prof, and no problem. We also met with math professors whenever possible, to discuss fit (or just get a feel for the department). A math prof at Case Western (he was great), told my son not to go to one school that had accepted him. He said their math department wasn’t as good as his or Rutgers (his other top options at the time).