What doies the evidence support? How does the claim withstand testing? Is there a simpler solution that explains everything just as well?
Credibility means how justified it is to accept the claim.
My claim that I had a sandwich for lunch yesterday is a credible claim because it's is reasonable to believe that I had lunch yesterday and it is also reasonable to believe that a sandwich is a common lunchtime food.
The claim that I skydived in the Alps yesterday is a less credible claim, since I live in Australia and our travel is quite restricted at the moment, and even if it were true, I would be unlikely to have returned home by this time.
Thanks, that's a start. Still, I don't see anything in this I could apply to the pool scenario to explain why you would reject the document or why you would consider the witness unreliable. I have your statement that a document is no evidence in and of itself, but I'm still left asking: Why? It leaves you incredulous. Why? What is so extraordinary about someone claiming the table was set? Why only the 2 possibilities? Isn't there also a 3rd possibility an accident of some sort caused the layout (rather than an intentional break)? And maybe a 4th possibility? Where do we stop?
In formal debate, such a thing is not allowed. It's not considered a valid argument that you're creative enough to come up with additional possibilities, and then dismiss a claim simply because other possibilities exist. You need to present a criticism that can be demonstrably answered with additional data.
You're close to doing that with the Alps example. A formal argument is laid out like this:
Kylie: I was skiing in the Alps yesterday (claim).
J.B.: What do you have to support that?
Kylie: I have a photo (data) of myself dressed for skiing standing against a snowy background (warrant).
Note that data is not evidence. Evidence is the combination of data with a warrant. A warrant is a means of connecting the data to the claim. The warrant, then, is that snow and proper equipment are required for skiing. You have demonstrated meeting those requirements, and so have a warrant to connect the data to the claim.
J.B.: That could be the Snowy Mountains, which is easier to reach in a day than the Austrian Alps (rebuttal).
Note the form of the rebuttal. It's not just, "That's too incredible to believe," or some vague, open-ended "show me the evidence". It's a request for data in a form the claimant can answer. A statement has been made that if you can show the location in the photo is not the Snowy Mountains and that it is possible to travel from Austria to Australia in one day, your claim will be accepted.
Kylie: Here is my date-stamped lift ticket with the name of the resort where I was skiing and my date-stamped plane ticket.
J.B.: Those are faked.
Now the objections are getting a bit ridiculous. Why would you go to the trouble of faking those items? It is acceptable to turn the tables on such a rebuttal.
Kylie: Explain why I would fake them ... with data and a warrant, mind you. Or, you could pay to have the tickets analyzed for authenticity.
CF does have a means (rarely utilized) for formal debate. But, for the most part, CF will always, always fail at this point. There is no way to hold people accountable - to make them demonstrate how seriously they mean their rebuttal by putting up capital of some kind.
All you've really got is your own judgement of your opponent's sincerity.