(For large mode area fibers, that task is simpler, but on the other hand the angular tolerances are tighter.) One usually does the alignment while watching the optical power throughput of the fiber with a photodetector at the output fiber end, assuming that only light launched into the fiber core can reach that detector (which may not be true when the fiber is rather short).
In practice, it is not always easy to align the optics for efficient launching into a single-mode fiber.
One may even have difficulties finding the position of the often rather small fiber core to which the input beam must be focused.
Intermodal dispersion can of course not occur in single-mode fibers.
This is an important advantage for the application in optical fiber communications at high data rates (multiple Gbit/s), particularly for long distances.
The launch efficiency under non-ideal conditions, namely with a wrong laser beam size or wrong angular alignment, can be calculated with equations as given in the article on fiber joints.