A zebrina or also known as blood banana will have red coloring on the fold of the leaf and more red on the leaves. Look at the leaves of your banana in the center where each side grows from. If it is red, it is a zebrina. Cavendish have a yellow-green stem fold.

google image is your friend! Musa Zebrina:
http://images.google.com/images?svnum=10&hl=en&lr=&q=zebrina+banana

And if your still not sure look at the bottom of the leaf. Zebrina will have a red tint to the underside of the leaf whereas cavendish will not. At least that is the way mine have worked so far.

Also, look at the petioles and general stature. Zebrina has skinny petioles and a thin and leggy posture.

Dwarf Cavendish has wide leaves and petioles and looks more stout.

This is a dwarf cavendish.
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