ANSWER: In regard to anyone seeing Lot's wife becoming salt we sometimes have to simply trust God and the Bible. We could ask a question like this one such as: Who saw God create the universe, all life and even the first man? Another Biblical example is that we have no written eyewitness accounts of a great fish swallowing Jonah or of the same fish being seen spitting him out on some beach three days later. It all comes down to faith.
Lot's wife did not merely take a quick glance behind her to see the destruction of the cities. The English word "look" used to describe her action in Genesis 19:17 and 26, comes from the Hebrew nabat (Strong's Concordance #H5027), which means to look intently at in a favorable manner. In short, she turned to longingly and (no doubt) regretfully witness the destruction of the places she really liked. Lot's wife's attitude about Sodom, and the willingness to minimize or overlook the gross sins and self-destructive behavior occurring where she resided, may have rubbed off on her daughters. It could have played a part in their willingness, just a short time after Sodom's demise, to do whatever it took to ensure they would have children (see Genesis 19:30 - 38).
Lot, not suprisingly, hesitated as he left Sodom. The two angels, who were sent to escourt Lot and his family out of Sodom before its destruction, had to grab their hands and lead them quickly out of harm's way. As they dashed out of 'sin city' the angels told the family 'Run for your lives! DON'T LOOK BACK and don't stop in the valley.' (Genesis 19:17). When the family reached the small town of Zoar, just as the sun was rising, 'Suddenly the Lord rained burning sulfur on the cities of Sodom and Gomorrah . . .' (Genesis 19:24). It was then that the wife of Lot turned to look at what was happening and became a pillar of salt (verse 26).
Lot's wife stopped running away from the city just long enough to turn around and view the destruction of where she use to live. As she stood amazed at what she saw, many Bible commentaries agree some of the burning or molten sulphur (translated as "brimstone" in the KJV Bible) likely fell on her from the sky and crystallized her into a salt pillar.
It is entirely possible Lot may have seen the demise of his wife. It is only natural for the person in the lead to glance back to insure the rest were following him. Unfortunately, this loss was not the end of his trials and troubles. Right after God's judgment of his mate, his two daughters carried out a misguided plan they thought would insure the continuation of the human race (Genesis 19:30 - 38).
26. Now as it was in the days of Noah, so also shall it be in the days of the Son of man.
28. And it was the same way in the days of Lot: they were eating, they were drinking, they were buying, they were selling, they were planting, they were building; 29. But on the day that Lot went out from Sodom, it rained fire and sulphur from heaven and destroyed them all.
32. Remember Lot's wife (Luke 17:26 - 29, 32, HBFV).