Re: Topsides going from 'White' to flag blue
The best sense for checking for fairness is your fingertips - far more sensitive than your eyes. Doesn't work for long waviness though. If the surface is such that you can't "see" any waviness, sometimes carefully laying a long strip of tape can help define them. You have to be careful to keep the tape "straight" because any wows in the line can look like waves in the hull.
In the end though, the best thing is to sand with a longboard when you are prepping for paint. A true longboard will be at least 6' long and require 2 or 3 people to handle it. That's what you have to do if you want a flawlessly fair hull.
If you're willing to settle for a little less perfection, you can use the pictured board sander, available from auto body supply shops along with the strip sandpaper for it. It's a one man tool and if you sand correctly (crosshatch on the diagonal) it will give you a very good surface - better than the majority of boats out there.
Using alternate colours of primer helps reveal highs & lows as well - a bit like using differing bottom paint to reveal when it's thin.
I, myself, personally intend to continue being outspoken and opinionated, intolerant of all fanatics, fools and ignoramuses, deeply suspicious of all those who have "found the answer" and on my bad days, downright rude.