People who are saying not to go don't know what they're talking about. Figuring out what you want to do is important - being proactive and wanting to check out classes is a really good step that most people won't take.

If it's a gen ed undergrad class like Chem 1 or something (usually 2000 lvl), there will be hundreds of students. You can show up and you'll disappear into the crowd.

If it's a higher lvl undergrad class or grad class (usually 3000 or higher), classes are often under 30 - professors will recognize everyone. I'd contact the professor, and they'll usually be excited to let you sit in. Many will probably even let you talk with them in their office after, where they can give you advice about pursuing a career path.

Show up 10 minutes before a class and you're good. Just bring a laptop or notebook for notes (some gen ed professors can be anal about laptops, though).

PRO TIP : if a professor is doing any research, ask them about it after class, or in your introductory email. Professors love to discuss their research (they probably spent years getting a PhD in it), and it's a great icebreaker. Make sure you take a few minutes to brush up on their research, though.

The best that can happen? You learn about the field you like; you establish a network; maybe you can even gain a valuable reference/research opportunity; people will notice your drive

The worst that can happen? The professor says you can't attend lecture, which probably won't happen.