I like your white top, but the sheer back coupled with opaque front would be a deal-breaker for me, so I understand your hesitation.
As far as telling the difference between "good enough" and "really good" items: you are on the right track if you find you want to wear it right away. The other trick I use is to say to myself, "if in doubt, go without". If I have any doubts at all, I don't buy it.
This is different, however, from buying something new -- a new silhouette, say -- as an experiment. Sometimes I like the idea of something, but I'm not sure it'll work for me. Sometimes the only way to know is to try it. I'll use the example of cropped, raw-hem, higher-rise jeans. I liked the look on other people, but I knew from past experience that higher rises weren't my favourite. The combination of cropped length with higher rise made it look more current to me, though, so what I did was go out and find a pair of relatively inexpensive jeans as a gamble, and cropped them myself. I was prepared for failure ahead of time.