Most of the nobles wouldn’t have thought to even ask. They’re good at seeing what they expect to see. The prince of a great house using poison (suitable for cravens, eunuchs, and women) in a duel is unthinkable. Oberyn’s lifestyle seem to consist mostly of swaggering right past other peoples moral codes and daring them to call him on it.
The people who might have thought of it couldn’t ask for political reasons. Oberyn is brother to the prince and the most beloved man in Dorne. They couldn’t demand to check his weapons without accusing him of being a coward, making Dorne their enemy. And that’s if they DID think of it, it didn’t occur to Tyrion until after he saw the spear, if I remember correctly.
As for the gods, even assuming they get involved in these things, which isn’t a sure bet, they’ve got some outs. First, Oberyn mostly won. The Mountain was disarmed, literally pinned to the ground, and for all practical purposes out of the fight, until the Viper decided to swagger into grabbing range. So maybe he actually won the duel, and was killed afterwards.
Not good enough, you say? It was a fight to the death! (Was it?) Fine. The Mountain was dead the moment the Viper cut him the first time. As of that point, his divine protection was gone, and it was just two guys killing each other.
It was lose/lose for the Lannisters. Mountain wins, Dorne is enraged. Viper wins, the Tyrells are enraged. Worked out about as well as Tywin could have hoped for, really. He needed the Tyrells more than he needed Dorne, a reluctant ally at best. That said, Tywin had recently said he had no intention of giving up Clegane. He was going to frame the conveniently dead-by-bear Lorch for Elyia.
There have been five trial by combats that I can recall,
Bronn/falcon guy: Innocent guy cleared
Sandor/Barec: Innocent guy cleared (He did do it, and enjoyed it, but it was legally correct. the Lord of Light seems like the kind of god that would consider that the** best kind** of correct)
Oberyn/Mountain, innocent guy found guilty, but with wiggle room
Dunk/Trial of Seven clusterfuck: innocent guy cleared (He’d done what he was accused of, but it was justified)
Dunk/Lucas the Longinch: Who the hell knows. Spider Lady might have done it, it might have been a ‘who will rid me of this meddlesome priest!’ situation, or it might have been an accident.
That’s well within the bounds of ‘the gods don’t care,’ but not conclusively. It may also be that the Lord of Light cares, when his avatar is involved in the fight, but the Seven don’t have that power.