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You would have to average the reading over at least several seconds but the non-invasive way would be to monitor the remaining fuel level rather than fuel flow. I think commercial units work by measuring vacuum at the air intake rather than fuel flow as these are closely related.
There are other methods, such as Doppler measurments in the fuel pipe which you can do with RF or ultrasonics but they are quite complicated technologies.
I'm not sure of the exact relationship but I would guess it relies on the greater air demand from an engine under heavier load (bugger gulps!) and as fuel is drawn in on the air stream it has a relationship with fuel flow rate. Possibly the vacuum measurement and engine revs can be used together in a calculation. Whether it applies in the case of fuel injected gasoline or Diesel engines I could't honestly tell you.