Sebastian is right.
There is no way that I know of to know what the variety of cacao is just by looking at it. Size is not an indicator - either very large or very small.
Color doesn't say anything as there are "Trinitario-hybrid" varieties (I am thinking of the acriollados in Nicaragua) that can be 10-30% white or pale (lila).
Aroma and taste are no help either.
That said, if you have been working with the same beans from multiple origins for a long period of time you can learn to distinguish between those reliably. However that won't help you when it comes to other sources.
clay - http://www.thechocolatelife.com/clay/