This would be my own opinion. I have only learned Taichi for 2 years and I am too amazed with how diverse is Taichi. Even with the standardization of the martial arts into 5 major group, there are even variation when it is taught by different teachers.
My take is to consider a few factors to decide on its authenticity:
For instance, Yang Shao Hou was famous for his fast or small/medium form, those descend from his lineage are likely to inherit that form.
The form should derived as a result of practicality. For Martial Art, its basic purpose is for fighting and self-defense, while health is only secondary. For instance, Yang Lu Chan never promoted his Taiji for health, his taiji is well known because of its effectiveness in subduing his opponents. Has Tung created the fast form because he found the form effective in its application?
Even when I began my lesson with my Taiji teacher, I researched into the history of Taiji, this also include reading up on the works of past Taiji masters.Of course what you need here is your own judgement of authenticity of their works. My take is that earlier documentation would be more authentic as there are closer to the source.
My first intention of learning Taichi was in 1997 when I worked in China. I have bought a VCD on Taiji lesson due to limited resource. The VCD was narrated by a person called Li De Yin. I have not watched the video until 2005 after I really picked up Taiji with my Teacher.
The form was meant for competition and I believe it was demonstrated by a champion of the competition. The stroke was beautifully played out. However, a question came to my mind, can it be used? The more I watch the video the more it convinced me that it is more decorative than practical.
My Taiji follows the lineage of Yang Shao Hou who passed his skill to Wu Tu Nan. Wu's occupation was a scientist while part time teaching Taiji, he has very few students, in one of his books he regarded Ma You Ching as his only deciple.
He has also documented his moves in the sixties via help of a movie camera. Someone has placed the clip on Youtube and I am allowed to witnessed how it was played out. So when my teacher teaches me the skill, I can reference this to his records.
This is just my share, I understand that you may have your method of judgement. End of the day it all depends what you are looking for in learning Taiji.