Here are a few different ways that your lawyer can help you to prove liability:
Look at the police report from the accident. This will usually contain information about who was at fault for the accident.
Review video footage or any photos you took of the accident, if there is any available. All of this evidence can be used to help prove that the other party was liable for the accident.
Examine any witnesses to the accident and see what they have to say about who was at fault.