How can you tell?

You may occasionally see groups of people performing tai chi in public.

How do you gauge the quality?

Look to the Tai Chi Classics for guidance:

http://www.newcastletaichi.co.uk/Taijiquan%20Classics.htm

Do not assess the tai chi on the basis of personal preference:

http://www.newcastletaichi.co.uk/Howcanyoutell.htm

Opinion is worthless.

http://www.newcastletaichi.co.uk/Opinion.htm