Well, the pump provides the suction, so a heater hose connected near the pump would draw coolant from the heater core.
The hottest coolant is taken from the cylinder head, so that hose delivers coolant to the heater core.
Following these hoses back to the heater core will tell you which is the heater core inlet.
As far as the rear, the hoses will be tied into the front heater hoses. At the rear heater core I don't know, unless you can physically follow it with your eyes.
I know this isn't the short answer but I hope it helps.
Also knowing Envoy or TB and what year and engine might help with a short answer.
4.2 engine
#3 is out of the head, so that is the hottest, flow is out of the engine.
#7 is at the back of the water pump, so that is suction, into the engine.