How can I figure out if my store bought chlorine is the percentage it says it is?
Bleach solutions can be analyzed by addition of iodide and back titration with thiosulfite - somewhat difficult for non-laboratory conditions. Kits are available, but expensive (~$250).

A less precise (but probably adequate) method involves measuring the heat evolved as bleach oxidizes acetone. https://www.flinnsci.com/api/library/Download/650c3e8ed0574ed18c7b37b8832cccae It seems that you might be able to modify this process to do a rough quantification (+/- 10%) quickly. It was also discussed briefly on Quora: https://www.quora.com/When-happens-when-acetone-and-bleach-are-mixed

Just realize that if you drop solid hypochlorite into pure acetone, it could explode, catch fire, and make you wish you had never tried it. Creep up on this reaction: add only small amounts of acetone (1-2 grams) to a dilute hypochlorite solution (~5% ?) until you get the feel of how fast and how hot this reaction will go.

Eye protection is mandatory. Also have some water available (like a hose). Just be safe by expecting danger.