First, I believe a very important question to ask is "what is the composition of the tea". There are 4 common oolong varietals in Taiwan oolong, Qing Xin (Green Heart) Oolong, Jin Xuan (Golden Lily), Cui Yu (Green Jade) and Si Ji Chun (Four Season Spring), and this is also roughly the sequence from most expensive to least expensive (but price also depends on leaf quality, and Jin Xuan and Cui Yu have similar price range).
In near past, if a Taiwan oolong is named after its production location, for example, Dong Ding, Alishan, Lishan... then Qing Xin Oolong, the most prestigeous varietal was by default used. Products of other varietals would be labeled with their varietal names. It was not law kind of thing, but pretty much the unwritten rule. Therefore in the past, I wouldn't feel comfortable asking a vender if his Dong Ding or Alishan was entirely composed of Qing Xin Oolong. Of course it was supposed to be Qing Xin, therefore the question, I think, would sound offensive. But in recent years, I've seen more and more Dong Ding and Alisha composed of all 4 varietals. It seems such a common practice that even some good Dong Ding and Alishan are made from different varietals, not just Qing Xin.
And here comes the problem - I did find larger amound of Si Ji Chun in less tasty oolong products. I personally don't like Si Ji Chun, not that it's a "bad" variety. Following photo A shows a winter Si Ji Chun with very young leaves, and it's actually a very pleasant tea. But when older (larger) Si Ji Chun leaves are blended in oolong products, very often, they add in significant bitter flavor in the tea, and sometimes entirely shade out flavors of other tea leaves.
Of course this is just my personal preference. But if you have a particular preference in Jin Xuan or Cui Yu, it would also be helpful to know the composition of the tea.
Among following photos, "A" is a Si Ji Chun. "B, H, I" are Alishan Oolong with blend of different varieties. The rest are Dong Ding Oolong with blend of different varieties.
When you buy tea product of specific varietal, you can recognize features of its taste and tea leaves. Here, "A" is an example of young Si Ji Chun, but there is a bigger leaf there. Usually the bigger leaves show more distinguishable feature.
I didn't start to collect photos like this until recently, so don't have a photo sample of product with Qing Xin Oolong only. Actually I started to wonder about this only after I found most Dong Ding and Alishan I've got these days are blends of different varieties.
Not all leaves in my photos show typical features. But the easily recognized ones include:
Qing Xi Oolong: E2, F3, G3, H1
Jin Xuan: E3, I2 and an example here
from a pure Jin Xuan product.
Cui Yu: B1, B2, B3, D1, D2, G1, I1.
Si Ji Chun: All in "A", C4, F2
A. Si Ji Chun winter harvest: