High-quality green beans should be plump, fleshy, and juicy, and should not have tough fibers when snapped. Also, check if the surface is a tender green color and free from insect bite marks, as these are details that require careful observation when selecting green beans.
There are many varieties of green beans, and the differences between them can be quite significant. Generally, long-pod (tall-stem) varieties are of better quality than short-pod (short-stem) ones, and round-pod types are superior to flat-pod types. However, there are exceptions, such as the tall-stemmed, flat-podded, white-shelled variety from Northeast China, commonly known as "you dou" (oil bean), which is tender, glutinous, and delicious.
However, green beans are prone to developing rust spots on their skin during storage, which is a sign that they have aged and their quality has deteriorated, making them unappetizing.
In conclusion, when buying green beans, you should choose those with a vibrant green color and firm pods. These will be fresher and taste better.