I speculate that they do it with a combination of senses, but primarily by feel and deduction.
With any two legs on a strand of web, they could instinctively deduce where the third (or fourth) foot should probably land in order to contact the strand. In addition to this, they have hairs on their legs which may help them pinpoint where the strand is (because the strands are basically linear, except near the junctions).
As for junctions in the threads, they may have a bit of memory regarding their own web structure and they may have a bit of a view of their web "horizon" along with general "feeling around" with a leg to see if you hit a junction thread.
This question reminds me of a story by Naturalist named Loren Eiseley
. See below...
Eiseley is hunting fossils on a rainy morning in his beloved Badlands or Wild Cat Hills. He encounters a hug orb spider tending her web in the buffalo grass. He touches a strand of the web with a pencil; the spider tends her guy-lines, and tries to read the movements but the message is incomprehensible.
A pencil point was an intrusion into this universe for which no precedent existed. Spider was circumscribed by spider ideas; its universe was spider universe. All outside was irrational, extraneous, at best raw material for spider. As I proceeded on my way along the gully, like a vast impossible shadow, I realized that in the world of spider I did not exist.
As he tramps on his way, he contemplates the white blood cells racing through his body, as indifferent to their host and ignorant of him as the spider is unknowing of the universe beyond her web. Thoughts rush in on Eiseley and the reader: the evolution of human life, the tenuous hold of the individual and the species to a place on the earth. Then, Eiseley offers his own analysis:
I saw, at last, the reason for my recollection of that great Spider on the arroyo's rim, fingering its universe against the sky. The spider was a symbol of man in miniature. The wheel of the web brought the analogy home clearly. Man, too, lies at the heart of a web, a web extending through the starry reaches of sidereal space, as well as backward into the dark realm of prehistory. His great eye upon Mount Palomar looks into a distance of millions of light-years, his radio ear hears the whisper of even more remote galaxies, he peers through the election microscope upon the minute particles of his own being. It is a web no creature of earth has ever spun before. Like the orb spider, man lies at the heart of it, listening. What is it we are a part of that we do not see, as the spider was not gifted to discern my face, or my little probe into her world?