There are many ways to tell Number 1, the 1847 stamp, from Number 3, the 1875 reproduction, but the simplest way is to look at the point where the top of the frill of the white shirt meets the oval frame. On the original stamp it meets the oval between the "F" and "I" of "FIVE", and on the reproduction at the top of the "F".
The engravers initials "RWHE" are much clearer on the original than on the reproduction. In this instance a proof of Number 1 was used for illustration, note that the issued stamps rarely have this level of detail.
The reproduction is slightly shorter and wider than the original and the reproductions often have a rich red brown color. Number 1 is collected in a variety of colors, ranging from red brown and even a bright red orange to a dark blackish brown.
Finally, the reproduction was never valid for postage and one would expect to see most examples without cancellation. In the truest sense, Number 3 was never a postage stamp, never valid for postage and of course not even an 1847 stamp, it just looks a lot like number one.