Detectives can gather evidence at the crime scene, gather testimony of witnesses or technical experts in various fields. The can use theories of science, even developing them on their own and testing them, or using a theory created by another, such as Newton or Einstein.
They can then present all this evidence in a courtroom. NONE of it is a proof. NONE of it is science itself.
That's why we have juries. They are tasked with weighing this evidence, and deciding which is valid and which is not.
In other words, let the accused go.