Draw Xray addon will help you to draw retopo mesh on top of your highpoly mesh, without problems of buildin blender xray (no proper backface culling).

How to use:

• grab addon from here Draw Xray
• install it by searching addon by name ‘Xray’
• go to Properties panel -> shading – Draw Xray (blender 2.79)

After installation you will be able to see your mesh now and few more control sliders:

• Object offset – will determine how much your lowpoly object will be rendered in front of other objects. Set it to 0 to get default blender behavior.
• Polygon color opacity
• Edge /Vertex color opacity.

Not tested