With one wheel stationary on an open diff, the other wheel should turn twice as fast. So 3½ turns of the yoke should result in nearly 2 turns of the wheel/axle for a 3.54, or under 1¾ turns of the wheel/axle for a 4.10. Or, differently, 1¾ turns of the yoke should result in nearly 1 turn of the wheel/axle for a 3.54, or just under ⅞ of a turn for a 4.10.
So raise one tire, engage the front axle disconnect (swap the vacuum lines and run the engine, or apply external vacuum), set the yoke horizontal, mark the bottom (south) of the sidewall, turn the yoke 3 half turns and another ¼ turn (1¾ turns total). If the mark ends up almost at the bottom (is pointing south), it's a 3.54. If it ends up turning less than ⅞ (is pointing SW or SE), it's a 4.10.