I guess just look really close at it.
I've wondered this too. I don't know of any way to ensure the bevel won't creep off center. Using something like edgepro only sets the angle and doesn't control how much of the knife is ground away. Constant checking of burr formation will help prevent overgrind. Further sharpening after the burr is formed will definitely be changing the bevel balance. When I sharpen (which is only freehand) I don't usually count total strokes but I won't do more than 10 passes at one "section" of the blade. My theory being the quicker I work from heel to tip should minimize inconsistencies. I can repeat the heel to tip routine as needed. Also about half the time I spend sharpening a knife is just on the tip.
Keeping an edge 50/50 is something I quickly gave up on once I experienced single bevel knives. Everything I use is right handed 60/40 or more. Ultimately, I have no idea how to know what the bevel is; I only know how I want the knife to cut. I use about 90/10 on my 270 Gyuto and equalize the bevel down as my knives get shorter to almost 60/40 on the paring knife. I don't bother with getting it to some perfect number. If I want more bevel on the right side I just keep sharpening the right side. If I want to center the bevel some, I will sharpen the left side some more. My main focus when sharpening is bringing the edge back to life with as few strokes possible making the knife last as long as I can. So moving the bevels toward/away from center could take months of sessions to finally get the knife where I want it.
This post probably didn't answer your question...So how do I know if my bevel is centered on the knife?