While Jack is in rehab, he tells a story of attempting to take his life "just shy of 13" years old by tying a belt to a ceiling fan (which came right out of the ceiling). The story is laughed at at the time of telling, but not soon after, the details of the story are called back when Jack is shown home alone with a belt in his hands. It's obvious from there what's about to happen, as Jack walks into the garage, sets his cowboy hat down, and takes a look at his dog who he'd just served a huge steak, clearly his way of saying goodbye.
There's a brief shot later through the garage window of Jack's torso swaying back and forth, and it's clear he's hanged himself.