Avalon Backflush (which hose is INLET)

[jerry3809]

2003 Avalon 3.0L
I know a backflush "T" normally gets inserted into the heater INLET hose, but on my 2003 Avalon both heater hoses terminate on the block. The coolant fill-cap is NOT on the radiator, and is mounted on the engine block instead. Neither heater hose goes directly to the radiator, so it's difficult to determine which is heater inlet and which is outlet. How do I determine which hose to insert the "T"? I've added backflush kits on every other car I've owned, but this is the first one I've owned that is piped this way.