Hey there,
As you may know many toys where also made of cast iron way back then. I know a few indicators for authenticating old cast iron toys that could also apply to your dog. First thing is, the makers always took pride in their work. They wanted to make good quality toys, and therefor get more money for them. Today's repros made in Taiwan and China are really cheaply made, and look very hastily put together. Also, old cast iron usually has a smooth feeling, while newer repros have a rougher texture. Another thing is to look at the seams were the two sides meet. If the seam is jagged and the pieces don't line up, it's most likely a repro, because again, old cast iron pieces were made well.
But just like Ivan said it's hard to tell without looking at it in person. Plus, these are general guidelines, and there can be pieces out there, old or new that don't fit these guidelines.
Hope that helps some.