It's in the growing plant that you see the results of virus infection, above all in a general weakness, poor flowering, lack of increase. It's much more often this poor growth that shows, than particular visible symptoms; lots of the viruses that do affect narcissuses don't actually have visible outward symptoms. Sometimes a narcissus can even tolerate one virus without being weakened, but often they get infected by more than one, in which case the general deterioration can get quite dramatic.
One or two individual viruses do have visible symptoms in growing plants, though. In narcissus, the commonest of these is probably yellow stripe virus, which may produce curly and/or finely streaked leaves, and/or streaky flowers, and perhaps deformed flower stems. Another warning sign is what looks like bruised purplish streaking up leaves or stems, fading to white, with the leaves then dying off - that's probably white streak virus.