An instructor acquaintance is very keen on his students not just putting the carb heat on and then off again blindly. Rather he want them to check the indicated RPM prior to the application of the carb heat, applying carb heat for a period (say 15 seconds), after which time, if the RPM has not increased, to push the carb heat in, and check the RPM as returned to it's original value.
If it returns to a higher value than it started at, you know you had some carb icing and that you should consider checking the carb heat more frequently.
He is also dismissive of carb heat application at less than Power Check RPM because at lower speeds the engine is not capable of producing very much of a temperature rise in the carburetor.