Instructions
Look at the base of the doll's head, where the hairline ends. There should be identification numbers stamped onto the back of her neck, denoting a true Sindy doll.
Look for the words "Made in England" stamped somewhere on the doll. Sindy dolls have only been made in England or Hong Kong, and only the ones made in England will bear the stamp. Sindy dolls have never been manufactured in China or the U.S., so a stamp that says such makes the doll a fake.
Look at the doll's eyes. A real Sindy doll will have real eyelashes, not painted on.
Examine her clothes. The original Sindy doll that was first released wore denim jeans, and a red, white and blue striped shirt. Finding one with its original clothes is a good clue that it is a real Sindy doll.
See if the doll has the original box with it. The original Sindy box was just big enough to fit her inside, pink in color and had a picture of a blond Sindy on the outside.