How can I check if there is enough inhibitor in the water?

My boiler was changed last June together with some rads. Previously the volume of water was 13 gallons so inhibitor levels could be calculated, but now it might be more. Consequently the amount of inhibitor required was guessed. If I take a sample of the water how can I measure it to see if there is enough unhibitor in it?

My new boiler is a Malvern 10/20 condensing boiler with boiler thermostat set to three quarters (70deg.). It is on an open system and there is sometimes a bit of kettling before it turns off. I don't want it to corode away. There were no kettling sounds during the first nine months.