The Letter Shin is easily distinguished by its three heads. The spacing of the Shin is something that takes practice for a Sofer to write correctly. The leftmost leg should have the head of a Zayin. (According to the Arizal,the left leg has the head of a Vov.) The head should have 3 Tagin on it, like a regular Zayin. The heads should not touch one another at all. Also, there should be no more than 3 heads in total, or the letter would be rendered invalid. The bottom should come to a point, and not be rounded or flat.