How do you determine which windings are field on a series DC motor?

Both the supply wires that enter a series motor is the field windings the field is split in two and is connected at the split to the brushes that is connected to the commentator of the armature, so the current pass thru the first half of the field then thru the armature and then thru the other half of the field. With a parallel DC motor the two fields is in series and the two wires is connected to the brushes so is the supply.