How do I know if someone else was legally at fault

The four elements of negligence
For someone to be legally liable, four things must be true: they owed you a duty of care, they breached that duty through their actions or inaction, their breach directly caused your injury, and you suffered actual damages. Each element must be proven. A person caused your injury doesn't automatically mean they're legally liable.
Duty of care examples
Drivers must follow traffic laws and drive carefully. Property owners must maintain safe premises. Employers must provide safe working conditions. Doctors must follow accepted medical standards. These duties exist because of the relationship between parties and legal expectations.
Proving causation
You must show the defendant's actions directly caused your injury, not something unrelated. If you were injured at work but the injury resulted from your own negligence rather than unsafe conditions, causation fails. Documentation from the incident scene, medical records connecting the injury to the incident, and witness statements all help prove this connection.