How can you prove fifty percent liability if you the police report says you are 100 percent at fault?

Liablity decisions are not based solely on the police report. Your insurance adjuster should investigate every aspect of the loss. If you want to provide details of the accident perhaps I could assist you with this. In short, how you prove fifty percent, is just that you (your adjuster) must prove that the other party contributed to this accident. Just because the police report says 100% does not mean that the claim will be settled that way.