The first thing to do, is unbreech the barrel. Looking through it, you should see light rings the length of the barrel. They will show you where they are bent, even if only a couple thousandths. If you aren't familiar with looking at the light rings, look through a modern shotgun, they are more easily seen, and will show what you are looking for. A rifled barrel has the same light rings, but are a bit more difficult for an amatuer to read due to the lands and grooves. I was barrel inspector for three major modern rifle companies, and some who worked there never could see them.
Once I establish where a bend is when building at home, I have a designated tree I whack them on until everything looks right.
When starting a new build, I straighten a barrel before doing anything else.
