I searched around, but I can't find it now. There was an excellent CSI parody script (very short) that seemed so apropos. I'll try my best to recreate it here:
Detective: We've got you cold on this one, Tommy. You're goin' down.
Tommy: Did you actually see me shoot him?
Detective: Well, no. But we have your fingerprints on the gun.
Tommy: Did you actually SEE me touch the gun? No? Then you've got nuthin'.
Detective: We also have your blood-stained clothes. The spatter-pattern is consistent with a close-range shot in the abdomen. Your victim was shot in the abdomen at close range.
Tommy: Did you SEE the blood spatter on me? No? Your so-called "evidence" sounds pretty shaky to me, Boss.
Detective: Yeah, I guess you're right. You can go.
The point is that evidence can be used to reconstruct past events, even if no one saw them happen. We accept that idea in almost every facet of our lives. We don't speak definitively about everything -- we can't say if Tommy was wearing a hat when he shot the man, or if he liked extra-cheese on his pizza -- but the evidence available CAN tell us something. (You didn't have to see me type this post to accept the idea that I posted it, for instance.)
At the very least, evidence can be used to discount theories. If the theory is that the victim was stabbed to death, the evidence of gunshot wounds would tell us that theory is wrong. Maybe he used a gunpowder-laced, bullet-shaped knife? Possible, but far less likely than the "gun" theory.
This helps to explain why scientists don't often speak in absolutes. The world just doesn't work that way. It's all about likelihoods, not "truth".
Dave