In the absence of grooves, this analytic geometry solution may work.

If everything is centered, the ends of the handle bars and the front axle will form a trapezoid.

Begin by verifying that the handle bar is centered in the stem. (having the same end to stem measurement on both sides.)

measure from the end of the handle bar to the axle. repeat on other side. If the measurements differ, it is not right, adjust handlebar position to minimize differences in measurement and repeat.

If you can't get it perfect, don't worry too much, small offsets are compensated for by riders with ease.

In the absence of grooves, this analytic geometry solution may work.

If everything is centered, the ends of the handle bars and the front axle will form an isosceles trapezoid.

Begin by verifying that the handle bar is centered in the stem. (having the same end to stem measurement on both sides.)

measure from the end of the handle bar to the axle. repeat on other side. If the measurements differ, it is not right, adjust handlebar position to minimize differences in measurement and repeat.

If you can't get it perfect, don't worry too much, small offsets are compensated for by riders with ease.