We've seen several instances of Trial by Combat in both the Game of Thrones TV series and the A Song of Ice and Fire novels. Trial by Combat is said to allow the gods themselves to determine the innocence of the accused party by having them win the fight if they are innocent and lose the fight if they are guilty.
But does it actually work?
In the Trial by Combat that takes place when Tyrion is accused of killing King Joffrey, his Champion (Oberyn Martell) is killed by the accuser's champion (Ser Gregor Clegane AKA The Mountain that Rides) even though we know that Tyrion didn't kill King Joffrey. That being said, Ser Gregor Clegane had already been poisoned by that point and his death was practically guaranteed - it could be argued that the gods stopped intervening at that point which led to the sudden reversal.
In the Trial by Combat that sees Lord Beric Dondarrion battle Ser Sandor Clegane AKA The Hound, Lord Dondarrion puts his defeat down to the intervention of the Lord of Light, talking about a whip of fire that shattered his sword.
Is Trial by Combat only ever determined by the abilities of the combatants, rather than the gods as people believe?