igor....By definition, topping is indeed checking a fuel flow limit against power achieved, but that limit may be preselected on the fuel control to protect an engine from Ng overspeed rather than an inability to produce further flow. It depends on the engine.
With a PT6T for instance, topping is adjustable, and a topping check is performed to ensure the Max Ng is attainable. On engines where Max Ng is not attainable without exceeding another limit (like on the PT6T-3D where the Ng limit is 109.2, and you'll ALWAYS hit Q or ITT first no matter how high you go), a part power trim stop is used to check calculated topping at a much lower value, but this test can't be done in flight.
Hope that helps a little....