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In The Goblet of Fire, Fred and George bet that Ireland would win but Viktor Krum would get the Snitch:
‘We’ll bet thirty-seven Galleons, fifteen Sickles, three Knuts,’ said Fred, as he and George quickly pooled all their money, ‘that Ireland win – but Viktor Krum gets the Snitch. Oh, and we’ll throw in a fake wand.’
How did they know this? (Surely they would have had to be fairly certain to pool all their money.)
Ok, I've found a quote from JKR:
ES: How on earth did Fred and George know that Ireland would win and Bulgaria would get the Snitch?
JKR: Well, I think that if you were really into Quidditch you could have predicted that. What they had -
ES: But how can you predict that, because you don't know when the Snitch is going to show up.
JKR: It was a risk. They risked everything on it. That is Fred and George, isn't it? They are the risk-takers in the family. You've got Percy at one end of the family — conform, do everything correctly — and you've got Fred and George, who just take a totally different life path and were prepared to risk everything. They risked all they had, which is as much as anyone can do.
They knew that Ireland was the better team and that Krum was the better seeker. That Krum would catch it, but not in enough time before Ireland had outpaced them.
So the twins bet on a long-shot outcome, based on both those facts, and the idea of pride--that Krum would be too proud not to.
The game ends when the Snitch is caught. They were betting that Ireland would be too far ahead when it was spotted. If it was spotted by the Irish seeker, they were betting that Krum would beat him to it, because he's the best in the world.
The odds were so far in Ireland's favor, that the point spread probably indicated that Ireland would be so far ahead that it would not matter who caught the snitch, and Ireland would still win.
They were betting that Krum would see that, and end the game on his own terms. Ireland may have had the better team, but Bulgaria had the better seeker.
It's likely that they looked at the spread, and at Krum's behavior over time in his games.
Most of the time, if the big point bump and game end caused by the Snitch being caught won't put your team over the top, you might try to keep the other player from catching the Snitch, in the hopes that your team will catch up enough, but if all hope is lost, you might catch it just so the difference won't be so much.
"What did he catch the Snitch for?" Ron bellowed, even as he jumped up and down, applauding withhis hands over his head. "He ended it when Ireland were a hundred and sixty points ahead, the idiot!""He knew they were never going to catch up!" Harry shouted back over all the noise, also applauding loudly. "The Irish Chasers were too good... He wanted to end it on his terms, that's all..."
The twins likely knew this from analysing the game and the players beforehand. Ginny does the same thing as Krum when she fills in for Harry in Order of the Phoenix, when Gryffindor loses to Hufflepuff by 10 points (chapter 26).
They did not know it for certain, it is simply what they predicted. That's why it's called gambling.
I think they got their hands on a totsy turvy time turny backy thing that Hermione had to go to the extra classes so that they could travel back in time. That’s how they knew the outcome. Full disclosure: my 10 year old figured it out and asked me. I checked the “internet” and no one came up with this explanation. My kid is a genius!
