I see that the Chinese Wushu Association has introduced a formalised assessment process to assign skill levels to practitioners. Given that it is all in Chinese, it's hard to actually see what the skill levels translate to. But Violet Li has a brief description, in her article about Grandmaster Chen Zhenglei being elevated to the 9th and highest level.
"The Duan Wei system is sub-divided into three categories: Elementary Duan Wei: One through Three; Intermediate Duan Wei: Four through Six; Advanced Duan Wei: Seven through Nine. For the Elementary Duan Wei and Intermediate Duan Wei, tests are required. Often times, practitioners need to score high in tournaments to qualify for a rank promotion. For the Advanced Duan Wei, practitioners need to have publication and/or research and exhibit significant contribution in promoting the art. There are also waiting times required between each Duan, for example, one has to wait at least six years before being eligible to apply for a rank promotion from the 6th Level to the 7th Level. With that, most masters who earn the highest honor of the 9th Duan have at least 50 years of experience in practicing the art and most of them earn their title in their 70’s or later."I personally believe that formalised panels of senior practitioners conducting an "examination" over a few hours is most likely to give you an accurate reading of your skill. After all, this is essentially the same approach that we use for all other qualifications, academic or otherwise (this is especially true of nonscientific disciplines).
Tai chi ability is in some senses relative to the skill level of your peers (certainly when we are talking about martial arts applications). So what better way can there be to measure your own level, other than be to be judged by tai chi "professors"?