One word: attachment.

Claims like, "The singularity will occur within this century," do not have attached implications, i.e. there aren't any particular facts we would would expect to be able to currently observe if they were true. Things we dismiss as bunk we either have evidence that directly contradicts them, (e.g. "The Earth is 6000 years old" is directly contradicted by evidence) or we lack evidence that would expect to observe with extremely high probability were they true (e.g alien abductions - it's rather bizarre that aliens would do such specific things and somehow invariably avoid large demographics of society. And plenty more... fleshing out this example isn't really my goal).

Bunk claims are thus those either directly and powerfully contradicted by evidence or that lack highly expected supporting evidence. Or those that are wholly unsupported by evidence and require some kind of magic to even be possible. I suspect that's about all there is to it.