Can you take a business class if you're not in a college's business school?

I’m majoring in History, but I’m also fascinated in Marketing. I was wondering if I could take courses in a business school so that when I choose to double major, I’ll be ready.

Yes, oftentimes students elect to minor in something out of their area of major. You would have to check with your specific institution as to the limits placed and courses required but it is very commonplace in college to do so. Sometimes you may even take a class at a neighboring college such as Harvard allows with MIT just to satisfy your curiosity. You could take a class that you’re simply interested in without the goal of even minoring in it. You are in college after all, it’s quite the place to learn more about things you think you might be interested in. Best of luck!