Are there many schools out there that will let you take a few graduate level courses so you can test the water without being enrolled? I've heard taking these classes can help boost chances of being accepted to graduate schools.
Most schools have some provision for this. Your best bet it to check out the school(s) you would be interested in attending if you decided to pursue a degree. That way your courses will count for something, you'll gain insights into the particulars of the school/program, and can sometimes work to help improve you application if you do apply for a degree program.
I would say the majority of schools allow you to take a certain number of credits as a non degree seeking candidate prior to enrolling in a program. And most programs allow you to transfer a certain number of credits from one program to another. Usually 9-ish credits.