How to measure heavy water concentration? regular tap water contains small %   D2O , let say a distillation process remove small amount of H2O . how can one measure this change using tools available to average person. 
 A: Apart from $D_2O$ (heavy water), natural water also contains $HDO$ (semi-heavy water) and of course $H_2O$.
The physical properties of these substances do vary from one to another, as can be seen from this Wikipedia comparative table. For example, the melting point of $D_2O$ differs sufficiently from that of $HHO$ or $H_2O$ to be measurable by a hobbyist or amateur scientist.
The problem is that deuterium ($D$) has very low natural abundance: only 1 in 6420 hydrogen atoms is deuterium (in the world's oceans), according to Wikipedia.
At such low abundance of $D_2O$ and $HHO$ the influence of these substances on the bulk properties of tap water is beyond reasonable means of measurement by an "average person". And a single electrolysis has only a small effect on these low abundances.
The same is true on the effect such low abundances (and any small changes thereof) on biological systems, even though much higher concentrations of $D$ does have a strong negative effect on the metabolism of most animals (like fish).
A: In "ordinary" water, about 156 out of 1,000,000 hydrogen atoms are deuterium.
If you were to pick any instant in time, these atoms would be arranged in water molecules as follows:$$HHO---0.999688024$$ $$HDO---0.000311951$$ $$DDO---0.000000024$$
So only about 24 molecules of true heavy water exist in a billion molecules of ordinary water.
In addition, these hydrogen and deuterium atoms are rapidly exchanged in water.  If you were to watch a particular one of the tiny number of $D_2O$ molecules, (don't ask me how) you would see the D atoms swapped out (and possibly back in again)
very quickly.
I don't know any "kitchen-sink" method to detect the effect of removing a few of the super abundant H atoms...
