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I'll tell you my method
1. Find the input if you barely don't know which one is input.
2. Use avometer as ohmmeter to check which one is short for each pins.
3. Ohmmeter is work as current source so when you find short one then it is the pair of input pin.
4. Power up the input with series resistor.
5. Measure the output which one is short so you now know how the current flow.
6. When you power off input that output pins become open.
Your optocoupler is work.
Take an analog multimeter, set it to the RX1 or RX10 ohms range and connect the black lead to the anode of the LED (pin 1) and red lead to the cathode (pin2) of the LED. The meter should register a low ohms reading, the exact value of the reading is not important, it should be low reading. Now just reverse the meter leads, there should be no reading on the meter, or it should show an open cicuit.