American Girl dolls and books were created by Pleasant Rowland, who founded the Pleasant Company in 1986. The original dolls were Felicity, Kirsten, Samantha and Molly. In 1998 the Pleasant Company was sold to Mattel, Inc. thereby making original Pleasant Company dolls a collector's item. There are many ways of telling the difference between a pre and post-Mattel American Girl doll.
Remove the doll's apparel to find out if the doll is signed. Pleasant Rowland autographed the torsos of several of the first three historical doll lines manufactured by the Pleasant Company -- Felicity, Kirsten and Samantha. An autographed doll is extremely rare and valuable. A non-signed doll does not mean the doll isn't pre-Mattel, however.
Examine the doll's torso body cloth. White muslin bodies indicate the doll was made anywhere from 1986 until early 1991. Dolls with tan cloth bodies are not made before 1990.
Examine the doll's neck stamp. Until 2005 stamps on the backs of American Girl doll necks said " (c)Pleasant Company", after which many doll stamps read "(c)American Girl LLC." Older doll neck stamps look less well typeset and more uneven, and in some cases the letters are arched. Some stamps are large and some are small, and the copyright lettering may be placed above or below the company name, but both have uneven lettering.
Look at the doll's eyelashes and other physical features. Older dolls have longer, softer, lighter eyelashes than Mattel versions. The Mattel version of Samantha, for example, has more droopy cheeks, darker hair, lighter skin and darker eyes than the pre-Mattel version. The pre-Mattel version of Josephina has darker skin and thicker hair than the Mattel version, which also has shoestring ties on the moccasins as opposed to the original leather ties.
The Pleasant Company neck stamp does appear on Mattel dolls, often appearing with uniform lettering and additional letters and numbers underneath the company name.