*shrug* it's basically a given, to me. idk maybe i'm remembering wrong but I could of sworn that grrm confirmed that oberyn had poisoned tywin and he was going to die anyway.
Of the mountain of foreshadowing, references, and coincidences that support oberyn poisoned him the clearest to me is that tyrion knew he would be on the shitter.
It's like TV. Nobody uses the bathroom in movies/TV unless it serves a purpose or furthers the plot. It can be used to show how nonchalant someone is by having them whipping it out and pissing in front of someone. It can be used to show someone is drunk pissing in their closet, etc, etc. It happens most often in comedies cus dick/fart/shit jokes are funny. But just random hero of the story taking a dump and that dump having no impact on the story? It doesn't happen. He wouldn't write paragraph after paragraph about how frequently Tywin is seen heading off to the shitter or how long he was spending in there, cus who wants to read about that. But he can insinuate that's what's going on by tyrion's thoughts, knowing he'd find his father sitting on the shitter. Some people think "oh he just had sex with Shae of course he's in the bathroom", idk about you but I usually have to piss after fucking, not sit on the shitter long enough for the girl i was with to pass out.
Also grrm mentions all these exotic poison's he's invented for a reason. He describes what they do and their effects so that the reader can recognize what poison was used to the characters he kills with it. Tears of lys on Jon Arryn, The Strangler on Joffrey, Widow's blood on Tywin.
Even with all the care that was taken with his body it still rotted and festered unnaturally, creating a gagging stench, etc. He was clearly poisoned. The evidence that Oberyn is the one who did it is pretty solid as well.