Depending on where the pressure regulator is, it may not matter which is flow & return? e.g. on a 916 the regulator is in the tank and so the fuel can flow either way through the injector bodies as it is simply a 'loop' - that said space constraints means they only really fits one way round, the original hoses should have an M (Flow) and R (Return) on them somewhere.
Otherwise power it up from a battery with a small amount of fuel in it - will be obvious which is the flow!
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M900, 916, LeMans II.
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