We stopped at rows for each of the 5 varieties: Cabernet Franc, Cabernet Sauvignon, Merlot, Malbec and Petit Verdot. I learned some characteristics about the buds/leaves as well.
Cabernet Sauvignon is the last to bud and the last to be harvested.
Merlot leaves are said to have a "big tongue", a large, wide leaf
Petit Verdot has a whitish film on the back of the leaves
Malbec has a very distinct, round leaf shape which helps to recognize this varietal.