How to estimate hydrogen peroxide concentration in an easy way?

I have bought 20 Liter of Hydrogen Peroxide about 2 years ago which I was told that it was pure at that time. I'm aware of instability of H2O2, which easily decomposes into water and hydrogen. 


The H2O2 at my premises had probably been decomposed partially within this period. According to its current practical effectiveness, I guess the concentration is still above 15%, but I can not be sure. 


How can I estimate its current concentration with using equipments at home? 



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> Note that I can find a small batch of known 3% H2O2 solution for some kind of comparison, if needed. 
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One path to safely determine the strength of H2O2 is:


* First dilute it to 1/3 strength (this is for safety when dealing with higher concentrations of hydrogen peroxide).
* Slowly titrate against a known concentration of NaOCl until the vigorous oxygen liberation ceases.
* One can also collect the volume of O2 evolved.


Reaction equation:


$\ce{NaOCl + H2O2 -> NaCl + H2O + O2 (g)}$


So, moles of oxygen liberated equates to moles of hydrogen peroxide. Also, moles of sodium hypochlorite employed should also equates to the moles of H2O2.


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