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{EDIT}Corrected an error in the procedure as originally written.{/EDIT}
There is a way to test the strength of bleach, but it isn't really all that simple. You need to dilute the bleach by 10,000 to 1 and then test the diluted bleach. One way to do the dilution is to add 10 ml of bleach to 1 liter of water, mix throughly, then add 10 ml of that to another liter of water, mix throughly and then test the FC level. The two liters of water need to be chlorine free, commonly distilled water is used. Also, the 10 ml measurement needs to be as accurate as possible, typically done using a syringe or pipette.