Yes, the lie detecting witch is Lady Amber of Stone Ring Keep. She is described as:
Ariane’s breath caught. Warily she eyed the Learned lady of Stone Ring Keep, who could discern truth simply by touching someone.
In chapter 28 Ariane confronts the knight Geoffrey who raped her. Geoffrey has been trying to provoke her husband into a duel, but this threatens an uneasy peace. Ariane requests Geoffrey be questioned "in the Learned manner":
Sir Geoffrey has insulted the honor of Lady Ariane. The lady has most forcefully requested that the issue not be solved by test of arms, for such would only jeopardize the peace that the Glendruid Wolf has worked so tirelessly to
A murmuring went through the assembled knights. Each knew what was at issue. Each had wondered why Simon had not challenged Geoffrey ten days ago, nor any day since.
Now they knew.
“Instead,” Meg continued, “Ariane requested that Sir Geoffrey be put to the question in the Learned manner. Lady Amber has agreed.”
However Lady Amber does not hold Geoffrey's hand but instead touches his cheek:
Amber took a breath and let it out slowly, composing herself. Then she rested one fingertip on Geoffrey’s cheek just above the place where blood had been drawn by Erik’s knife.
As soon as Amber touched Geoffrey, she went pale. Sweat stood clearly on her skin. Her eyes were so dilated they were almost black. Only her clenched jaw kept her from crying out. Whatever Amber sensed of Geoffrey when she touched him was intensely painful to her. Yet touching Geoffrey was the only way Amber could learn his truth.
I'm not sure I'd call Lady Amber a witch though. She is a Learned and while gifted they are not really witches. When Geoffrey insults her by calling her a witch she responds:
Without a word Amber reached inside her tunic with her free hand and drew out a silver cross. Bloodred amber gleamed at five points of the cross that lay nestled in her cool hand. Her fingers closed around the cross for the space of four slow breaths, then opened again. There was no mark anywhere on Amber’s hand, no sign that the cross burned in protest at being held against her skin.
Geoffrey looked to the lord’s table, where Blackthorne’s chaplain sat.
“What say you, chaplain?” Geoffrey shouted.
“Have no fear of Satan within this keep,” the chaplain said in a voice that carried easily the length of the great hall. “Lady Amber is like Lady Margaret, strangely blessed by God.”