Yes, but it's a pretty subtle difference.
For example a US Diver's Conshelf looks a lot like a Scubapro Mk 2 in general layout even though the Conshelf is a balanced diaphragm reg and the MK 2 is an unbalanced piston reg. Similarly, the balanced piston design first stage is normally "L" shaped with the long part of the reg being oriented 90 degree from the axis of the yoke connection, but there are several exceptions to this with severa l "L" shaped diaphragm frst stages.
Consequently the most reliable visual difference between a piston and diaphragm reg is the location for the holes that let water into the ambient chamber. In a piston piston first stage the ambient chamber is between the piston head and the yoke/DIN air inlet for the reg. With a diaphragm reg, the ambeint chamber is located on the end of the first stage opposite the Yoke/DIN inlet.
So for a piston reg, the ambient chamber holes are usually in the middle of the regulator body while on a diaphragm reg, they are on the end (or as in the case of the Conshelf, it's just a slot in the end). You also may not have a hole if the reg used a sealed ambient chamber design. So on a diaphrgam reg, you'd look for another diaphrahm on the end or a rubber cover on the end with holes in it. A piston reg on the other hand may have a rubber seal or band around the middle.
And of course there are regs that will be hard to tell. The Scubapro Mk 2 for example, since it is an unbalanced piston first stage, has a very short compression chamber and the holes to the ambient chamber are closer to the end than to the middle again making it look a lot like a diaphragm reg in general layout.
However, if you know the name of a first stage, this link may help as the type of design is usually given.
http://www.scubaboard.com/showthread.php?t=86200
