The large summer-time fruit of Common Blackberry (Rubus allegheniensis) is familiar to most nature wanderers. But even without the dark juicy fruit to aid in identification, this plant can be recognized with relative ease.
As you surely know if you've spent any time gathering wild Blackberries, these plants are well-armed. The stalks have serious prickles and demand caution and respect from gatherers. Older stems are not round in cross-section, but instead feature several flattened or grooved sides.
Common Blackberry has five-parted palmately divided leaves, and often retain a few dead leaves into winter. Palmate describes an arrangement of leaflets arising from a common center – similar to how your digits connect to the palm of your hand.
Members of the Rubus genus often hybridize, and certain species can be difficult to distinguish from one another in the field. While the features I've described here are useful, if you seek positive identification, I recommend consulting a reliable plant guide. For New England plants, I use the recently published Flora Novae Angliae by Arthur Haines (2011).