It is easy to distinguish female plants of the species in the summer, fall and winter by the position of the flowers and fruit. Flowers and fruit are at the leaf axils on Oriental bittersweet and are only in terminal panicles on American bittersweet stems. Also, the fall fruit capsule color is yellow for Oriental bittersweet and orange for American bittersweet. It is more difficult to distinguish male plants because they do not set fruit. Flower position is still indicative. During the spring, leaf unrolling is also indicative. Conduplicate (folded in half lengthwise with the upper side inward) leaves are Oriental bittersweet and involute (inward curling) leaves are American bittersweet. Other potential characteristics such as leaf shape (Oriental bittersweet has rounder leaves) and fruit number per cluster are highly variable and indefinite.
Oriental bittersweet leaf shape is highly variable and unreliable for identification
For more information on identifying bittersweets:
MDA Noxious Weed Program
County Ag Inspectors