1. Most mass made dolls have some sort of id marks on them. The majority will have a mark under the hairline at the back of the neck. You can usually start there. Check the body and the bottom of the feet also. There are reference books available, just like silver makers hallmarks that can identify the maker
2. Check out the size and the clothing. If it's original clothing you may be able to identify the approximate age and country of origin.
3. Once you've narrowed it down to manufacturer, try a doll club to get even more info. Don't forget to ask the owner any information they have. If they remember getting it as a child, you may be able to narrow down the date even further.
4. If you are totally stuck, or just feel it's too much work, there are doll appraisers on line that will appraise the value and give you info on your doll. They usually charge between $10-$20 per doll.
Hope this helps. If you want to learn more about collecting, check out my book on Amazon. "Cashing in on Collectibles"