I covered most of this in the Overheat Article, but I’ll cover it again here. The best way to find coolant leaks is with a cooling system pressure tester. This is a tool that you place on the opening of the cooling system, usually in place of the radiator cap, where you pump pressure into the system. Your cooling system is designed to hold pressure. If it can’t hold pressure, it probably has a leak. When you pressurize the system with the tool, the coolant will leak out where you have a pressure loss.
Most leaks are external, but sometimes there’s an internal leak. You use different testing to find these, but the pressure test will get you started. Much of this material is covered in the Solving Overheating Issues article. You might also find this video useful.