I'm feeling quite unjustifiably proud of myself right now. A language comm I follow had a "What is this language" post - the OP had listened to a clip many times and tried to categorise the sounds, but had failed to identify the language. So - without looking at the comments, obviously - I followed the link and listened to the thirty-second clip (once). Within five seconds I was going, "That's Norwegian."
Of course, being me, within ten seconds I was saying "But how can I know that?" Given that I don't speak Norwegian, I don't know any Norwegians, etc, etc. I don't know, is the simple answer. All I can say is that it was practically instant, and then I followed it up with some possibly spurious "reasoning":
(1) "It sounds kind of dark and Northern". Impressively scientific, yeah?
(2) "It's not Swedish". I've watched the Millennium films several times, so while I couldn't do the sounds myself, I remember enough to know that it sounded quite different to me.
(3) "It's probably not Icelandic". Because I was very struck to see how like Old English Icelandic looks and so I'm guessing I would be able to pick out the occasional word, which I couldn't here.
(4) "It's probably not Danish". On no grounds at all other than I'm assuming Danish sounds a bit Germanish.
Also, I remembered my OH, who is good at doing accents, reporting to me about five years ago what a customer had just said to him. The customer, an English speaker, was himself reporting what a Norwegian had said to him that day, in English with a Norwegian accent. A bit of a long trail, but for me it was the confirming factor.
So yeah, either I'm brilliant or I'm subconsciously remembering how Norwegian sounded when I visited the place for one week over twenty years ago, during which time no-one ever addressed me in Norwegian. In which case I'm - brilliant.