Based on the comment M. Farooq with respect to the oxidation-reduction potential (ORP) sensor, I would suggest a pure chemical test.

In an inverted container containing the subject treated water, to test for dissolved chlorine and hypochlorous acid, inject a volume of stabilized hydrogen peroxide.

The following reactions should govern the system:

$\ce{ Cl2 (d) + H2O (l) <=> HCl (aq) + HOCl (aq)}$

$\ce{ HOCl (aq) + H2O2 (aq) <=> HCl (aq) + H2O (l) + O2 (g)}$

The last is a fast reaction and I would rapidly note (by marking) the volume of oxygen rapidly released. Subsequent elevation in released oxygen could be produced by other catalytic agents acting on the H2O2.

I would recommend performing a trial run on distilled water to which is added a known amount of Sodium hypochlorite. Repeat with boiled tap water plus NaOCl and observe any growth in oxygen release with time.

Based on the comment by M. Farooq with respect to the oxidation-reduction potential (ORP) sensor, I would suggest a pure chemical test.

In an inverted container containing the subject treated water, to test for dissolved chlorine and hypochlorous acid, simply inject a volume of stabilized hydrogen peroxide.

The following reactions should govern the system:

$\ce{ Cl2 (d) + H2O (l) <=> HCl (aq) + HOCl (aq)}$

$\ce{ HOCl (aq) + H2O2 (aq) <=> HCl (aq) + H2O (l) + O2 (g)}$

The last is a fast reaction and I would rapidly note (by marking) the volume of oxygen rapidly released. Subsequent elevation in released oxygen could be produced by other catalytic agents acting on the H2O2 itself.

I would recommend performing a trial run on distilled water to which is added a known amount of Sodium hypochlorite. Repeat with boiled tap water plus NaOCl and observe any growth in oxygen release with time.