I almost hate to admit this, but I don't worry about it that much when reassembling a steam loco model,
other than to make sure that when the counterweight it at 12 o'clock, the eccentric crank is pointing "mostly straight up".
Sometimes it's at 10 o'clock, sometimes it's at 2, and sometimes, the crank points straight up to 12.
If the crankpin is keyed, as Peteski and others have pointed out, that usually makes the decision easy because
if you don't get it right, the wheels can't make it around without locking the rods.