The head tefilin are supposed to go above the hairline, or above where the hairline was on someone whose hairline has receded.
The former situation is easy to deal with. A man whose hairline hasn't strayed from its original location can simply find its location every morning with his fingers.
Do you have any tips for how a man with a receded hairline should find this location? Are there any clues available to the fingers? I'm looking for practical advice, ideally advice that can be implemented without ready access to diagrams and mirrors.