What two elements normally must exist before a person can be held liable for a crime

1 - performed an illegal act (actus reus)

2-while performing the act, had the required intent or specific state of mind (mens rea)

Though there could be a lot of things that an officer can check out to see if a person is held liable for a crime or not. But two main things could be; An actual Crime (theft or murder etc) and an Eyewitness.

In case of theft, the things should be lost, which will show that a theft has taken place. And an eyewitness like a person who watched the person doing the crime.

In case of murder, there must be a dead body, the murder weapon and an eyewitness. Murder weapon is necessary to make sure that the person who is being held liable for the crime has done this murder and someone saw him doing the act.