The first thing I'd recommend doing is to buy (they aren't expensive) a cooling system pressure tester. These are hand held pumps that screw onto the top of the radiator tank in place of the radiator cap. Depending on your cooling system you may need an adapter to adapt the pressure tester's cap to your radiator cap's threads. Pressure test the cooling system first on a cold engine. Pump the pressure up to the rated system pressure which is typically 15-16 PSI gauge. Next inspect all around your radiator tank seams, hose clamps, water pump pulley shaft, underneath around the freeze plugs, where the heads meet the engine block and around the intake manifold. If some of these terms are unfamiliar a quick Internet search will get you up to speed. You may need a small inspection mirror and mag light flashlight to see some of the areas listed. If nothing turns up cold, then check again when the engine is hot. Safety is very important here. When the engine is hot the cooling system is pressurized and severe burns can occur if you remove a radiator cap from a hot engine. You can remove some of the coolant when the engine is cold so that when it heats and expands it won't overflow the radiator. Then, leave the radiator cap off and run the engine until it is hot (or mostly hot). Monitor the radiator while the engine is heating up and if you see coolant expanding and rising up as if it is going to spill over then screw on the pressure tester and turn off the engine. If you turn the engine off as the coolant is expanding but, before you put on the pressure tester the coolant will spill over. Pressure test as before. You've now checked the cooling system for external leaks. Note that it isn't uncommon on an older engine to have more than one external leak and a large leak will mask or hide a smaller one. So, every time you fix an external leak or leaks re-pressure test to verify all of them have been fixed.
If no external leaks are found then, you may have an internal leak. You may buy a kit which consists of a glass tube with a suction bulb on one end, a cone shaped rubber piece on the other end and special fluid. The fluid goes into the glass tube. You get the engine hot with the radiator cap off as described above and put the cone shaped tube into the radiator filler neck. Then, you squeeze the suction bulb to draw radiator vapors into the fluid. THIS IS IMPORTANT. DO NOT draw radiator liquid into the tube; vapors only otherwise, you'll contaminate the test fluid. So, it is important to remove some coolant from the radiator before getting the engine hot; as before. This can be done with a turkey baster.
If the test fluid changes color, then there is exhaust gasses in the radiator coolant indicating a failed head gasket or a crack in the head or block (not too common) so most likely a head gasket.
Another place for an internal leak is water to crankcase. This could be evidenced by the oil on the dipstick being white and a foamy to creamy consistency. Think of oil and water shaken together.
There is also the smell test. If you can smell a sweetish scent coming from the exhaust when the engine is hot this also indicates that coolant is leaking into the combustion chamber but, this should be verified with the test fluid mentioned above.
Is the carpet on the passenger side floor wet and/or does your windshield steam up when you have the defroster on. If yes to either of these then you may have a leaking heater core. This may not show up with the pressure test unless you can remove the covers from underneath the dashboard and inspect around the heater box seams with a flashlight.