It appears that neither you nor the other party involved called the police. This is unfortunate because police officers usually can help assign blame from the placement of the vehicles and their experience with the direction of dents, etc.

Some insurance companies will have an adjuster or other personnel that is experienced in examining a dent and giving an analysis such as you want. I would contact your insurance company, which you will likely have already done anyway to tell them of the incident, and see if they have an adjuster that can help you in this way. If not it is possible that you can contact or hire an accident Reconstructionist that is an expert in this field.