If the other spring looks like it has been replaced, but no one knows, I usually ask the custome to decide. Newer springs usually look black. Older springs have an orange tint from rust. Unlike springs, cables do not normally wear at the same rate. Some cables rust at the bottom from exposure to salt only on one side of the door.
Many cables fray because of a rough spot on only one of the cable drums.
I never recommend replacing both cables unless I see a problem with the other cable.