Definitely the front feet x-ray.
Good friend found a really nice Andalusian mare, (I know, not a pony, but same metabolism issues) but she was a bit chubby, nothing frightening.
Owners said she’d never foundered, they’d had her from weaning.
There were some photos of her chubbier, but again not awful, no giant cresty neck or anything.
I insisted that she get blood tested for insulin resistance (came back negative) and x-rayed for coffin bone rotation. The x-ray came back positive…that was a horse that could be managed, but would need careful oversight so as not to founder again.
And I do believe the owners never knew she foundered, she was sound enough at the vet check, but obviously her coffin bones had rotated.
If the vet check had been just blood work, flexions, friend would have spent a LOT of money on a very high maintenance horse.