Distinguishing a lead's turn from a follow's turn

How to tell apart a lead's turn from a follow's turn in partner dancing? How to signal which one you want as a lead, or interpret the signal as a follow?

Look at the hand and where the fingers point. In a follow's turn, the fingers point in the direction the lead is asking her to turn, and they will point outside the partnership on the same side as the hand. In a lead's turn, they will point across the partnership — e.g. the hand will be roughly in line with the lead's left shoulder and follow's right, but pointing to the right. See the illustration above.

(*From a lead's perspective, the circle is the follow's head and the top left diagram shows "asking the follow for a right turn," while from a follow's perspective the circle is the lead's head and the top left diagram shows "he's asking me for a left turn." But if the only question is "who's turning?" then the left-right distinction disappears — both perspectives agree who's turning.)

This convention follows from other conventions about turns. Conventionally, the lead's fingers point the way he's suggesting she turn. Also by convention, turns are under the arm. If the fingers point across the partnership, the follow can't turn in the direction indicated by the fingers and still go under the lead's arm. So, the way to indicate a lead's turn follows from other conventions.

Thanks to Karen Malak, who tried to explain this to me when I first started but I didn't get it, Lydia Emeric for explaining it in her Beginner East Coast Swing class Wednesday, and Scott Hader for asking the question that I have been wondering about but didn't ask.