We measure liters per minute for HHO gas with a meter made especially for HHO gas (little pricey). I see a lot of people using meters in videos etc. that are made for air. This is not accurate HHO gas weighs differently.
If your hydrogen generator is efficient and you are certain you are not creating steam, this is probably the best way to go without spending money on a HHO gas meter.
Take your HHO gas out hose while running your engine and let it feel up an empty liter bottle, while timing it with a stop watch and then doing a little math.