This is a valid concern. Some thoughts on it:
With vigorous flow, the H2O2 concentration does not remain at the initial dose for the entire treatment time. It will be reduced not just in reaction with algae, but also quite rapidly at first in easier reactions with omnipresent bacterial films and organic wastes. So this cannot be compared exactly to a medicinal dip, or any H2O2 treatment where little or no flow is present.
Because of this, in a tank with good flow, the dosage has an exponential rather than a linear effect.
Both of these react fairly rapidly, peroxide is completely gone in less than 1 hour and most of it gone in 5 minutes. I have a Glut test kit, just have not gotten around to making the decay graph curves in a REAL planted tank, I would describe is more as a non linear curve, than exponential.
You'd need very good current to make sure things are homogenous if you took a sample of glut, but you could take several with a turkey baster type of syringe at various points. It'll last longer than H2O2.
If someone wants, I have a spare Glut Hach Test avail
You'll need a hot plate and a stirrer as well.