Refractometry seems like a straightforward approach? http://www.h2o2.com/technical-library/physical-chemical-properties/radiation-properties/default.aspx?pid=59&name=Refractive-Index

I usually measure H2O2 concentration by density. With a digital density meter that reads to 4 decimal places of g/cm3, this is quick and easy. I'm using it for solutions around 30 % ("100 volume") - density a little over 1.1 g/cm3.If you're using markedly lower concentrations, say below 5 % (density just below 1.02) then the precision may not be good enough for your application.

Tabulated data is available in Easton et al -Trans Faraday Soc 1952, 48, 796.