Remove any external casing on the diode. You will need to see the shape of the circuit itself to determine whether your diode is well or epitaxial.
Orient the diode so that the aluminum is facing upwards (with regard to your orientation; i.e. if you have placed the diode on a table to inspect it, orient the aluminum away from your body). If you cannot identify what material on the diode is aluminum, know that the side with the aluminum will have a gap in the middle.
Follow, with your eyes, the convex area at the bottom outward to a chosen side. This area may either be a well (for well diodes) or a layer (for epitaxial diodes). Both wells and layers are in roughly the same shape. Since the diode is parallel in structure, it does not matter which side you choose. Follow this area until it ends (i.e. evens out).
Observe the area past where the convex layer ends. If this area is all one area and has no other material after it, the diode in question is a well diode. If this area is roughly flat, is not the end of the diode and has more material after it, the diode in question is an epitaxial diode.