The acknowledged way of determining a Master Teacher's authenticity is through tracing their Reiki lineage back to the founder Mikao Usui. 

I have three lineages, from three different Reiki Masters:

1. Mikao Usui - Chujiro Hayashi - Hawayo Takata - Virginia Samdahl - Dr. Larry Rosencranz - Lisa Lee - Paula Bojsen

2. Mikao Usui - Chujiro Hayashi - Hawayo Takata - Convoluted lineage of William Rand - Paula Bojsen 

3. Mikao Usui - Kan'ichi Taketomi - Kimiko Koyama - Hiroshi Doi - Richard Rivard - Mark Brewer - Paula Bojsen

All of these trainings and attunements have been in-person - not obtained by internet, not by DVD, and certainly not "self" done. As you can see, the third lineage is unique in that it is NOT of the Hayashi and Takata lineage, but is Japanese. Hiroshi Doi brought Reiki that has clear Japanese roots to the West in 1999 at the invitation of Richard Rivard.

Lineage detail:
* Mikao Usui: Creator of the Reiki system in Japan and First President 
      of the Usui Reiki Ryoho Gakkai (Reiki Japanese Association founded 
      in 1922).
* Taketomi: Third President of the Gakkai, initiated by Usui.
* Koyama: Sixth President of the Gakkai, initiated by Taketomi.
* Hiroshi Doi: President of the Gendai Reiki Healing Association, present 
      member of Usui’s Gakkai,and initiated by Kimiko Koyama. 

What is important about this last lineage is that in order to teach this system - Gendai Reiki - you are required to teach it exactly as it was taught by Hiroshi Doi. (In order to do that, you have to take his class, it's not something you learn from a book.) This is not true of other Reiki teachers in general, who are free to add or subtract what they feel is or is not relevant.
I am on the right.
I am on the right.
I am also trained as a Reiki Master Teacher by William Rand of the International Center for Reiki Training.

Check out my name on this list from the Center

Sensei Rand is a world-renowned teacher of Reiki, and has written several books, including The Reiki Touch, and collaborated on books such as The Spirit of Reiki with Frank Arjava Petter and Walter Lubeck> Sensei Rand also developed Karuna Reiki, and Reiki healing attunements.