I'll trust them once I know they are good but I like to check them out first.
I've tried the string thing and the board thing but by far the easiest to check option is just to clamp together some boards, rods, or whatever to form a [ shape and use that as a caliper to compare one side to the other. Measure from the center of the swingarm pivot axle to the wheel axle. Get them even to within 1 mm or less and then check the hash marks for accuracy. From there you can use the markings with confidence.