Police forensic science is useless by that logic. The police don't observe that a guy was murdered, they just take samples from which, by observation, we may reasonably infer as much.
No, it is not "useless." It is not an exact science either. Police, witnesses, juries, and all can be fooled precisely because they enjoy neither direct observation of the events of which they treat nor direct insight into the motives of the accused.
Nevertheless I trust police forensic science which treats of a narrow line of questioning far more than I trust the typical evolutionist who accepts a tale regarding a 4.5 billion history of unobserved events and then goes on to invoke the law of the land to support his story exclusively as "science."