You can use the standard ...
Published by Joseph Cotruvo, President at Joseph Cotruvo & Associates, Water , Environment and Public Health
You can use the standard water chlorine measurement methods, but with DI water (no chlorine demand) and high sample dilution and back calculation. Typical laundry bleach is in the 5% hypochlorite range; hypochlorite marketed for water treatment is often in the 12 to 15% range.