Blend shape & MFnBlendShapeDeformer

Hi,
I am developing a plug-in and would like to get access to a blend shape deformer via API. I noticed there is a class named MFnBlendShapeDeformer but actually I don't know how to get the information wether a shape has a blender shape connected to or not. If I had to create the blender from scratch I would be able to make use of the MFnBlendShapeDeformer methods but in my case I would like to be able to connect a blend shape to my plug-in and get access to the shapes that were used to create the blender. Is there anyone who can help me?

Thank you, mikelem