From easiest and less is the following. A consideration many overlook is the radiator cap itself. If it does not maintain a specified pressure the system will run hotter. When water is under pressure the boiling point is raised. Many parts stores will test the cap for you for free. Caution is strongly advised to only remove the cap when the engine is cold. Best have a professional do this.
A result with a bad cap is water heats more quickly and its counterpart steam occurs at a lower temperature. Usually it will spill over into the coolant reservoir tank. When it is steam it simply dissipates making detection more difficult. At times the coolant reservoir will be a dingy dirty look like there was muddy water present if the radiator is afoul from lack of maintenance.
If one is constantly filling with a lot of water there most certainly may be a leak. The proper way to check the system is a knowledgeable person with a pressure tester. The system is pressurized and then leaks can be sought. Some common ones are the hoses best checked first. A next best look is at the water pump seal. Freeze plugs are more of rarity today, yet can be considered. A good look about is needed overall.
Another that can be considered is the heater core. Usually if that is the case then the floor carpet will be getting damp or wet on the passenger side. An intake manifold leak results with water in the oil pan. One trick as a partial check for that is to look at the dip stick. If the oil is a muddy brown there is a chance there is water in the oil.
A worse case scenario is a head gasket leak or cracked cylinder head. To check those properly is to do a combustion leak check. That is done with another tester. It is basically is a litmus test. A chemical is placed in the checker, inserted at the radiator opening, and if it changes to a color while the engine is running. That is a result of combustion gasses present in the cooling system. Then there usually is a bad head gasket or possibly a cracked cylinder head.
The above of course has the normal disclaimer. Most certainly a professional must be consulted for diagnosis and repair. However, the knowledge with above offers understanding.