Measure the voltage drop across the LED in the forward direction, the so-called forward voltage. For this you need to place the LED in the forward direction so that she lights up. The forward voltage is about 2.0 Volt.
A normal silicon diode requires a forward voltage of 0.7 to let current pass. For a LED, the value is much higher.
One more feature the LED and silicon diode have in common. They have a very low internal resistance. Once the required forward voltage is met, the current through the diode rises rapidly with rising voltage.
A typical red LED however is damaged if currents exceed 20 mA. You therefore need to limit the current through the LED with the resistor R1...