Pat, I am very glad you posed this question. There are many of us out here trying to create and identify ways to do just that. Cis mentioned the book Parenting with PTSD which I co-edited. This is a revised edition of the original book because we started asking ourselves the same question. How do we know. We chose to include chapters in the new edition, written by survivors who provide trauma-informed family support services in hope that the reader/parent will gain perspective on what a TI service looks and feels like. Cis references some of the authors in her comment to you.
I have to say, I think preparing the parent to gain her own definition of what TI means will help as well, because although we (the ones supporting the parent or the parent) may not have a way to know if a service truly is TI when referring or entering the program, once receiving the service, if it doesn't line up with the parent's understanding of how they should be being treated and served, they can get out right away.
Please do continue sharing your thoughts on this topic!