First, you need a resistor to set a current through the LED part. The datasheet recommends 5mA. Use a 680Ω from a 5V supply. This takes into account the 1.6V drop across the LED. You can measure this voltage.
Then see if the transistor part is conducting current. The best way is to use a current meter as shown in the attached diagram. Then you can work out the "current transfer ratio". The 100Ω resistor is there in case the transistor is short-circuit. The datasheet says the CTR should be between 80% to 600%.
The current should stop when the LED is disconnected.
For example if transistor conducting, and suppose there is no resitor od 100Ω (mean that 5V is connected directly to colector(node 4) of transistor), in that case the transistor is short-circuit. Than what is happening? What is voltage at node 4?? It is indeterminable ! Meaning, is it 5V or 0V (GND)? To avoid that confusing state you must add a resistor betwen 5v and node 4, also that resistor will limit the current through transistor.