One of the most helpful things to look at is the doll’s neck stamp. This can help determine whether you have a Pleasant Company or Mattel American Girl doll. Use this information in conjunction with others details on your doll to help determine how old your doll is.
1990
The earliest doll I have is an early 90s transitional Samantha. (She is considered transitional since she has traits of a white body doll, but has a tan torso). This neck stamp was used in the late 80s to 1990:
1991-1994
The following neck stamp on this Molly doll was used from 1991 to 1994:
1995
This neck stamp is from a 1995 Girl of Today 6 doll. Notice that the copyright symbol is smaller than the one on the 91-94 stamp.
1996-1999
In 1996, the first machined stamp was used. All the letters are uniform and in a straight line. This stamp was used through 1999. Below is a picture of the stamp on a Girl of Today 6:
2000 (Kit)
For a quick aside, 2000 Kit dolls had a unique neck stamp that was only used that year. This stamp has a handwritten look to it like the earliest PC stamps and the “(c) Pleasant Company” appears boxed in. All Kit dolls were made by Mattel.
2000s
In 2000, the transition of Pleasant Company to Mattel was complete. The 96 to 99 stamp continued to be used, however Mattel stamped alphanumeric characters under the “Pleasant Company” stamp. This Girl of Today 21 has “68H9” stamped under the “Pleasant Company” stamp.
Here is another example from a 2001 Lindsey doll with “12H9” stamped under the neck stamp:
And, lastly, an example from a 2005 Girl of Today 13 doll with “66 C3” stamped under the neck stamp:
As you can see, all of these neck stamps have different letter/number combinations, but all are Mattel dolls.
Mid to Late 2000s
The “Pleasant Company” neck stamp was phased out in the mid to late 2000s as the head molds wore out and were replaced with new ones with the “American Girl LLC” stamp. There is no definitive year for when these PC neck stamps were replaced with the AG ones. Below is the neck stamp on a late 2000s Kit doll:
Find more information for determining the age of your doll here.