The various fields of modern magic furnish us with several approaches that, while promising, are ultimately imperfect means of attaining this goal.
A strong shaman, with an appropriately bound spirit of mind, can determine whether or not the witness believes his or her statements to be accurate. This is of no help, however, if the witness is misremembering, confused, drugged or not of sound mental state.
A wizard might scry the scene to view, across the veils of time, the actual event. The challenge for the investigator, in this case, is to find a way to catch the interest of one of the world's handful of wizards and survive the encounter.
There is a common misconception that the chosen followers of the Goddess Shonak-i are granted by their Lady of Justice the ability to glean the absolute truth of a witness' statement. The reality is more subtle - those devotees are given to know whether or not the Goddess Shonak-i believes the statement to be accurate. Unfortunately for the dedicated investigator, the Goddess' most common reply is "How should I know? I wasn't watching!"
This is why, when accosted by violent men, it is always advisable to whisper a prayer to Shonak-i to get Her attention."
from "A Guide to the Investigation of Misdeeds" by Murnau de Farrell