Switching from a 200 to a 180

I have an 04 600 and when I purchased the bike it had a 200 tire on the rear. Being the cheap ass that I am I decided to ride it out since it was practically brand new. Well now its finally worn down and I want to switch it over to a 180. Is there a way to measure the rim to make sure it is a stock 600 rim. The rims are chromed and I have no clue if the guy got the stock ones chromed or if he found some used ones from a 1000.