In the history of football, there are several matches that are recognized as agreements, although they have not been replayed. We are talking, of course, about major international tournaments, because there are much more of them in domestic championships. For the World and European Championships, for the Champions League and the Liberatodores Cup, match-fixing is, after all, a rarity. And, then, they resort to agreements, nevertheless, in cases where there is no even an approximate probability that this will be followed by exposure and punishment.
The most famous example of an agreement: the match between Argentina and Peru at the World Cup 1978. The World Cup was held in Argentina, and victory in it was a state task, so the ruling junta spent a lot of resources to help Cesar Menotti's national team. Argentina needed to beat Peru at least 5-0. The issue of the outcome of the match was decided at the level of the presidents of the countries. The price of the issue, as it turned out, amounted to several tens of thousands of tons of grain. Argentina won 6-0. To the credit of the Peruvians, I must say that they did not even try to pretend to resist.
This, again, is the loudest case, a global scandal. Further more difficult. Fixed matches, in due time, ruined “Marseille” Bernard Tapi. But there was one hundred percent proof. The Valenciennes players, who took a lot of money from Tapi for the surrendered match of the French Cup semi-final, then, for some reason, complained to the police. Perhaps they felt that their conscience was worth more.
Now about our days. How to calculate a negotiated match? First, you need to have a good idea of the history of opposition between teams in any particular league. Here's a simple example. The Russian Premier League is full of teams whose task is to simply exist. They will not fly to the FNL, but they will never get into the European competition zone either. Sometimes strange miracles happen with such teams. She plays very well in the fall, then in March and early April. And at the end of April it starts to "pour in". Loses to everyone in a row, including the recorded outsiders. It is curious that in the first round of the next championship she will easily beat all these teams. This system is sometimes called a wheel. You don't need anything, you drain the game to the outsider, help him survive, and next season he will pay you back. In parallel, it helps to make good money on bets. True, you need to bet carefully. Last fall there was a scandal with the Ural - Terek match, the coefficient for the away victory of Terek was very low. I don’t remember the exact figure, but there is such a coefficient for the victory of “Barça” over “Eibar” in a home game. Almost one to one.
Something like this, knowing the history of the opposition between teams, the coefficient of bookmakers and the real strength of the clubs, you can calculate a potential agreement. You don't even need an insider for this.
I think only insider information from the bowels of the teams. Which is little shared. You can also follow strange quotes from bookmakers, but they do not necessarily indicate a real collusion or drain. Occasionally there are over-obvious situations, like with the Denmark - Sweden match at Euro 2004, but this is rather an exception.