A method of ensuring the aircraft has sufficient fuel for the task, is operating correctly and that you know where you are.
All Aircraft
FUEL - Check the quantity against the expected quantity at that stage of the flight. Consider this as an amount up or down and compare that to the wind you have experienced so far. Also ensure that the fuel tanks are in balance and correct with fuel transfer if needed.
ENGINE - Check the engine instruments for any fluctuations or unusual readings.
ELECTRICS - Check the generator is providing charge to the battery.
LOCATION - Check your location is as you expect and you know where the nearest air station is in the case of an emergency.
Modern Aircraft
The additional systems of a modern aircraft often require a little more than just the above FEEL. Consider this as FEEL+ and can include things like:
HYDRAULICS - Check the Temperature and Pressure
OXYGEN - Check the contents, flow and pressure
CABIN - Check the Pressurisation and Temperature
Combat Situations
Again, Combat situations may require further checks, see Fence In and Fence Out
Repetitive
These checks should be conducted about every 20 mins (as a minimum after each 30 mins) or after each change of altitude or turning point of a navigation exercise.