Your first stop in engine identification is the VIN that will supply you with a code to identify the engine with which the vehicle was originally equipped. If you believe the engine is not original equipment for your vehicle, identification can be a little more difficult. Check the labels found on about the engine compartment (radiator shroud, power steering pump reservoir, air cleaner) for more clues. Your last resort is the often difficult to interpret engine codes.
All engines will have some engine build code attached on a label and usually stamped or laser cut into a machined pad on a portion of the block.