Look for white smoke from the tail pipe. (More than what's normal in winter).
Unexplained loss of coolant from the reservoir with no obvious external leaks. Air bubbles going into the reservoir while the engine is running. It looks like the coolant is boiling but there's no steam or excessive heat associated with the bubbles. You may see that too before the engine has even warmed up, so even though it may look like it's overheating, you know it isn't, ... Yet.
Overheating engine. Combustion gases can pool under the thermostat preventing it from opening. They open in response to hot liquid, not hot air.
Your mechanic can draw air from the radiator through a glass cylinder with two chambers partially-filled with a special dark blue liquid. If combustion gases are present, the liquid will turn bright yellow.
If the leak is relatively slow, you can add a small bottle of dark purple dye to the coolant, then search in a few days with a black light. The dye will show up as a bright yellow stain that you can follow back to the source. Suspect a leaking head gasket if you find the dye inside the tail pipe.
If the leak is real bad, you may get "hydro-lock" and the engine will not crank. A cylinder will be filled with coolant which doesn't compress like the air normally does. If the engine turns over normally, you don't ave hydro-lock, but coolant can still be leaking into the cylinders, just at a much slower rate. Hydro-lock is pretty rare. You typically have to get the problem fixed long before the leak gets that bad.