There are three main ways to make out the difference between the inlet and outlet of the heater core. The first one is that the inlet usually has a piece of technology called the heater control valve. This is responsible for controlling the steam released by the coolant. Another way to tell the difference is that when the heater is turned on, the hotter of the two hoses will be the outlet. The third way to check which is the inlet and outlet is to follow both the hoses. One of them will connect to a small metal pipe called the water pipe. The other pipe is connected to the engine and is placed horizontally. The pipe connected to the engine is the outlet.