With the serpentine style belts on mercruiser engines, there is usually a sticker on the front area of the engine with a belt routing diagram for your engine model for you to follow along during belt installation.
As Don said, serial number... Try looking down at the block above the starter motor. Should be an aluminium tag riveted to the block with the serial number on it.
Is this engine in front of an Alpha One drive or a Bravo drive?
