I usually ID this based on the red fruits, straight spines/ thorns, and the surrounding environment and plant community. However, in the past, I've not bothered thinking about the difference between Common barberry (Berberis vulgaris), which is native to Europe, and Japanese barberry (B. thunbergii). One of the key differences is that Common barberry has sharply toothed leaf margins, while Japanese barberry has 'entire leaf margins' (no teeth). It's late Autumn in NY. Looking at this picture, it's clear that leaf characteristics are not going to help me.
Another trait difference is that Common barberry as 3-pronged spines. I didn't recall seeing this in the field. When I zoom into this picture, it becomes pretty blurry, but I'm pretty sure that each of the spines is by itself. So I'm going with Japanese barberry as my ID.
Both Japanese and Common barberry are in the Berberidaceae family (i.e., the barberry family). There's some nice information on this family both in the above referenced Wikipeadia page and in eFlora. Seems to be a fairly species rich family (ca. 650 species), but relatively few here in northeast North America (based on Flora Novea Angliae).
Sources used:
previous knowledge
Flora Novea Angliae
USFS Weed of the Week information sheet (Common barberry)
