Four or so years ago, I posted my email address on the Internet so people struggling with figuring out what American Girl doll they own could send pictures of their dolls to me so I could identify them. Every so often, I get emailed photos of dolls that are 18 inch dolls and seem like they could pass for American Girl dolls, but they aren't American Girl dolls, so I figured I'd post an easy guide to how to tell if you have an authentic American Girl doll:
First, if you can remember where you purchased your doll, that might help in figuring it out. American Girl dolls aren't available in toy sections in stores like Target and Walmart- Target carries Battat's Our Generation line and Healthy Roots dolls, while Walmart has the My Life As line in stock. American Girl dolls are primarily available on the American Girl site, the American Girl stores in malls across the country, and at select external sites like Amazon, Kohls (2016-2018) and Toys R Us (2016-2018).
If you can't remember or just want an easy guide, that's fine as well!
First, check the doll's neck. If it says "American Girl" or "Pleasant Company", then the doll is an authentic American Girl doll.
Another way to check the doll is to check the doll's body for a body tag. Body tags are more prominent in Mattel-era dolls, but they exist:
Hope this helped!