Question

How was the poet able to make out that it was some sad song sung by the solitary reaper?

Answer 1

Because the song that reaper sung is of sad tune and by hearing that song the poet thought that it was a sad song related to a war in the past, some natural happenings, death of some lovely person.
And the reapers voice was so sweet that the poet compare her voice to the voice of cuckoo bird as the bird is known for her melodious voice
Hope it helps