Use the correct hose. Fuel line may not fail, but why risk it? Possible failure modes could be swelling or leakage at fittings. Those hoses rarely need replacement anyway. If you really need to know which is supply and return, I'd guess the one on the driver side is supply - temporarily disconnect it, run the end into a bucket, let the engine idle for a few seconds and confirm that tranmission fluid is pumped out. Then top up the fluid with the transmission at operating temperature. If the fluid were pumped out of the open end of the cooler in the base of the radiator, would mean the other hose was the supply (pressure) line.
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