|You really can't in the winter. Older p.i. plants can have thick stems and are viny.You have to look at more of the characteristics of the plant and go from there for winter identifying. I know where some small vines were in the summer and can barely identify them. But i do know where some huge vines are and they are quite identifiable.
Check on line for pictures, etc... and get familiar with all the similarities. It may help you.