To find any coolant leak sources, you need to check all the components of the coolant system for any discolouration left by any leaks or signs of weeping. This is probably best carried out on ramps. The radiator, oil cooler, water pump, thermostat, hoses ,etc all need to be closely examined. There is a dye that can be added to the expansion tank to help make any leaks more visible. If you search for “coolant leak dye” on the internet you should find more details of the dye. I would imagine most motor factors will sell it. The radiator expansion tank should be checked for any sign of scum or oily film on the sides of the tank. Any evidence here could mean you have a leaking head gasket.
Good luck.