Iphone 3G/3GS how can I tell the difference?

How can I tell whether my Iphone is a 3G or 3GS, I bought the phone second hand and now get the feeling that it is not a 3GS and I have been scammed, I just would like to know as Ill stick with it if 3GS if not then atleast I will sell it on truthfully, I read somewhere that if the 4th/5th serial number is between 40-45 its a 3GS or if its under then its a 3G but my 4th is a 4 and 5th is a 3 so hard to tell which is which. I did pick it up quite expensive therefore should be a 3Gs. Model number is MB489B, FW 05.15.04. Please help and if you need any other detail please let me know, thanks