I have my doubts that it's that easy to tell the difference -- the buoyancy of hydrogen w.r.t. air (1.12 kg/m^3) is only 7% more than that of helium (1.04kg/m^3). However, I think you're probably right that it's not pure hydrogen, on the basis of the flame colour.
I think you could tell the difference between a balloon filled with hydrogen and one filled with "balloon gas", the cheap option which is just enough helium to float and the remainder nitrogen. In the UK at least noone puts pure helium in balloons commercially. A H2 and N2 mix might work just as well and would reduce the available combustion energy.
Density of nitrogen: 1.165kg/m^3
Density of helium: 0.164kg/m^3
Density of balloon gas (70% helium, 30% nitrogen): 0.464kg/m^3
Density of hydrogen: 0.082kg/m^3
So for a 0.02m^3 balloon, the lift generated (assuming a zero mass balloon and 1atm pressure - not true, but fine for this comparison):
Balloon gas: 12.3 grams lift
Helium: 19.2 grams lift
Hydrogen: 21.1 grams lift
And with a 10 gram balloon, this is a pretty convincing result - 5 times the net lift for hydrogen over balloon gas.
Balloon gas: 2.3 grams net lift
Helium: 9.2 grams net lift
Hydrogen: 11.1 grams net lift