Well, ways to tell whether it is a good dance school... among others:
The teaching qualifications of the dance teacher, where they themselves trained, and what professional experience they have (teaching and/or performing).
The examining body the school is using - some are much more well-known than others and have a far more rigorous teacher training system & rules/regulations.
The number and level of classes offered and the facilities. Also, whether classes for the youngest pupils are taught by fully-qualified teachers, or by older teenage students/trainee teachers. This happens with depressing frequency, and they don't always have the experience or technical knowledge to train youngsters.
The exam results of the students, and what successes do they have in things such as auditions and being accepted for further training, particularly the standard attained by more senior students.
And whether students progress according to ability rather than strictly by age group/cohort.
Also whether there is an enthusiastic, positive, motivational ethos permeating the school - this is an elusive quality to pin down!
Since your dd's school is new then it wouldn't be all that easy to tell I suppose, but you can certainly check out the examining body and the teacher's qualifications & experience, and whether there is a positive atmosphere.