I believe the million dollar question here is really about how it was preserved rather than how it was distinguished. In the short video about Laetoli footprints, Dr. Lovejoy mentioned that Laetoli Footprints are virtually indistinguishable from the modern-day footprints taken from a forensics study. So basically, he is hinting at the fact that most of the hominins that we are familiar with today exhibit the same two-foot upright walking characteristics, and it was highly likely that it remained this way for the entirety of its existence that is, some three million years ago. With that, we were able to conclude that Laetoli Footprints are from human-like ancestors. Nonetheless, the way how the volcanic ashes eventually condense into tuffs and ultimately into moldable muds that have withstood some three million years of weathering is nevertheless extremely impressive. At the end of the day, though, these findings are crucial because they would solidify the fact that human-like creatures existed some three million years ago during the Pliocene Era.