I'm sure someone will post pictures soon, but until then . . .
If the face of the hammer is flat, it is the original, not converted.
If the face of the hammer has a "step" in it, it's a conversion hammer.
Also, of course, in a converted gun there will be a transfer bar device down in there between the hammer and the firing pin, as c.r. mentions.
Edit . . . Dang it c.r. you're right about that. The Super Blackhawk conversion hammer is just a regular flat-sided Blackhawk style without the nice wide thumb piece of an OM Super hammer.