The Focus Settings has “Draw Debug Focus Plane”, which color codes 3 main aspects of the DOF. Have you read the docs for DOF yet? I suggest doing so, and while reading, keep the editor open with those settings visible. I read it all without doing that a few times, and I keep forgetting some of the settings and what their functions are.
In addition, in the Camera Focus Settings, two of the settings are disabled and the docs probably give instruction on how to get those enabled to tweak them (Current Focus Distance and Current Horizontal FOV). It’s also probable that the two sets of settings work in conjunction, but not sure how.