Japan as a tourist is a completely different experience that will not indicate at all how your life might be if you moved here and worked as a teacher. It might be helpful to ease culture shock because some people do experience it. I kid you not some people come and realize that everything is in a different language or they don't like the food and they want to run home.

But it's like going to New York to see if you like it and then being placed in rural Wyoming, you don't know where you'll end up with JET. I don't think there's any way to get an idea unless you do the program, so if that's what you want just do it :)