What does bad vs good look like

You can't tell much by looking at it, except if the boot is torn, which would allow dirt into the joint and ruin it in a short time. Mainly, the indication of a bad cv joint is noise, at a low speed in a turn, as going around a corner. A cracking sound will indicate that the joint has become contaminated, and needs to be replaced. That's a matter of replacing the "half shaft" axle with a new or rebuilt one (the cv joint is part of the half shaft assembly).