Driveshafts are built with the yokes at each end of the tube aligned. If the yokes are not aligned, the driveshaft is phased incorrectly. Driveshafts will always have all yokes aligned. Failure to phase and align a driveshaft properly will result in vibrations that will destroy the driveshaft, dyno, and/or engine, and improper phasing can cause serious bodily harm when the driveshaft reaches its failure point. Be smart. Be SAFE.
There are alignment arrows showing alignment between the two drive shaft ends. These are shown in the example below.
There is also an arrow stamped inside of the tube that should align with the other end of the driveshaft. This will indicate that it is in phase and aligned as it was when it was balanced at the factory.
Another way to ensure proper phasing and alignment is to look for the serial numbers. All serial numbers will be stamped on the same plane of the driveshaft. This also depicts that the driveshaft is in correct phase and that it is aligned as it was at the factory when balanced.