You can normally find the gas rate of an appliance in the manufacturers instructions.
Failing that you can work it out off the Gross heat input whether that be in Btu's/hr or Kw/hr.
If you don't have manufacturers instructions they will be online or there may be a visible data badge on the appliance.
I use a calculation from my head but it is round the houses and one I have used since God knows when. It is accurate but I dare say there are more modern ways even if they do get you to the same answer.
If you google Kw to M3/hr gas, there are a few dotted about that will work.