Past lives

 One of the best indirect ways of finding out about life after death is past life memories. Using legal principles, as many are from children under five, these both suggest they are not capable of making it up, and so many are corroborated by real places, people and stories I can't find a way to dismiss them. The best source is the book Before by Jim B Tucker, and was written up using scientific methods as he is a doctor.

Other explanations are telepathy, where the child is hooking into another person's records, but even so at the very least this confirms the ability to access such records, which I will describe next. 

Hypnosis is the quickest way to discover past lives, and besides being guided by another the method once hypnotised is similar to psychic readings just allowing what comes in. If you then find any supporting evidence, such as locations and historic records, you are clearly in the right area. The more you repeat the process the more should come, and the easier it can be through opening that part of your mind.

All these areas overlap. Children doing complex maths or music at the age of five, especially those with no training, have to have learned it somewhere. Also nearly everyone knows a few things naturally as a child they had never been taught, but most don't consider it as special or even realise they do. But we all have to learn advanced skills and insights somehow, and if they appear in young untrained children where else could it have come from?