How do you know who's a good offensive HC or OC?

It's a guessing game.

Then we can worry less if our HC or OC is a genius.

Though they're still only as good as their talent.

Fans seem to have opinions on these guys without a whole lot of information. How do they coordinate a practice? What goes on during team meetings? How well do they communicate with the players and the head coach? How good is their game planning?

But the best test I see is if you are ravaged by injuries can you be competitive. I know it is outside of the box, but I would ask permission to interview Tennessee OC Todd Downing. Despite the injuries that offense was good enough to get a first round bye. He also has a track record of making Derek Carr a Pro Bowl QB. He is also younger than you’d think (1980 birthday). Most recently before becoming Tennessee OC he was their TE Coach and you saw how guys like Jonnu Smith developed.

A good coach is a coach that gets the expected output or better from the roster he's given.

But you can have an idea. Especially if you know what you are looking at and looking for. Fans usually only see that they come from a good team and are usually more driven by emotion. Rarely do we see what goes on with these coaches off the field and and between games and that is a huge piece of their job.

Sometimes you have other coaches that help to prop up or work very well with the HC or OC. I think Shurmur would have had more success here if he was able to add Stefanski to his staff (another reason to not bring in Zimmer).

You can hire a great coach and have a great team and it may still not work out if the players don't match the scheme, injuries. Or sometimes you want to hire a coach because you think so highly of them that you don't realize that despite being great, they don't fit the culture, the structure or vision of the team.

I was impressed with how the Lions parted ways with Anthony Lynn their OC recently. He not a bad coach, but he wasn't the right coach for what they were looking to do. Everyone including Lynn saw that and decided to part ways amicably.

You can't get fooled by a great interview. You need to look at their body of work you need to learn how they do their job, how they make their decisions and why. And then you need to assess "is this the right person for our team right now?" and are we the right fit for them.
IMO that is where they went wrong with Judge. I think they loved his potential and over looked the fit

It all sounds like common sense, but you would be surprised how little that happens with hirings at any company
Sorry that got a lot longer than I thought

asking this similar question to a couple of former head coaches. They remarked that the better OCs (and DCs and for that matter) have the ability to construct a variety of gameplans throughout the season to be ready for each week's respective matchup.

And it wasn't discussed as just having a base one and then tweaking it. They really went back to the drawing board and were sharp enough instructors that they could get the players ready to perform to it in a short time frame.