Bleach in a pool I assume you mean amount of chlorine in swimming pools?
Chlorine treatment of swimming pools. Normally the the amount of Chlorine in the water is checked by test strips, which are normally part of a pool treatment test kit. The disposable test strips are dipped in the water and will change color, comparing the color against a test chart determines if more chlorine needs to be added. You will also need to test the acidity level of the water i.e. pH. The pH for pools is determined with another type of test strip in the kit.
Another Answer: Actually you can use common bleach in your pool as well if you like, although Chlorine tablets are the easiest and more economical to use. They dissolve slowly and maintain a more constant chlorine level in the pool. Also, an OTO test kit (which uses drops) is far more accurate than test strips. Hatawa.