Most successful method I have used is to add a "coolant dye" to the cooling system. Run the engine untill leak appears and test with a "black light"--- UV I guess. Will show the source in a bright color (usually yellow) and dye will wash off when work is completed. What you don't need is to pull the engine and still have doubt as to the cause. Works well with
oil leaks also but be sure to use the proper dye as it is different for most applications Good luck Ron