In order to find out what bootrom your iPhone 3GS is running you need to go to Settings>General>About. Once you are there, scroll down until you see the iPhone’s serial number and model number. Either one of these will help you determine which bootrom you have.
New bootroms have a model number that starts with MC and old ones have a model number that starts with MB. You can also tell which bootrom your iPhone 3GS has by the serial number. If the fourth and fifth numbers are lower than 35, then you have a lower bootrom version.
Source: iMore