The test psychologists use for this fits on the back of an envelope. Do it before you read what your number means.
Do the test first. Reading the explanation ruins the measurement.
You need a blank sheet of paper, a ruler, and a pen.
1. Draw a horizontal line across the page, as long as the ruler allows. Do not measure anything yet. 2. Put the ruler away and look at the line. 3. Mark where you believe the exact midpoint is. One mark, no adjusting. 4. Repeat on four more lines, drawn at different lengths, on fresh parts of the page. 5. Now measure. For each line, write down the true midpoint and your mark, and note which side you landed on and by how many millimetres.
Average the five errors, keeping the sign: left of true is negative, right is positive.
Most people with typical vision land slightly left of centre. The effect is called pseudoneglect, it is usually a percent or two of the line, and it has been replicated since the 1980s. The common explanation is that the right hemisphere handles more spatial work and gives extra weight to the left of your visual field.
Three things that shift the number:
Scanning back and forth. Sweeping the line repeatedly lets you count your way to the middle. Look once, then mark.
Using the paper edges. If the line is parallel to the page edge, you are measuring the page. Draw at a slight angle.
Adjusting after committing. The first mark is the measurement. A second one measures your patience.
This measures a one-dimensional judgment. It says nothing about how well you divide mass, which is the harder problem, because volume grows with the cube of a shape's size while your eye reads length. That is covered in why you cannot cut anything exactly in half.
If you want the mass version, cut a piece of fruit in two and weigh both halves on a kitchen scale. The gap between the two numbers is the same bias, in grams.