Is there a way to determine the orientation of a benzene-ring ethyl group in synthesis reactions?
Unless the temperature is really low, you will not be able to distinguish the two conformation isomers (conformers).

Rotation about single C-C bonds is rarely hindered. It can be, by using bulky groups or a tether, but neither is the case here. I expect the benzyl*-methyl bond and the phenyl*-ethyl bond to rotate freely at reasonable temperatures (say, above 250K).