If the leak is small it will only leak under pressure, that means when the engine is hot. The water will evaporate and/or blown away which makes is nearly impossible to find the leak. The only way is to find a small leak is by using a pressurise tester when the engine is cold, not just cooled down, but really cold.
I would not replace the radiator if the car is not overheating and you are not sure if it is leaking.