Is this an optocoupler ans how do I identify the pins?
Yes, it's an opto. You can confirm by googling the part number and the word "datasheet": "LTV4N35 datasheet", and find the datasheet for the part. https://www.arrow.com/en/products/ltv-4n35/lite-on-technology

One of the pages in the datasheet will show the pinout:


If you're slightly lucky, the datasheet will even have examples of usage in a circuit and which external parts it needs connected to the pins (like resistors, clock crystal, capacitors, inductors, etc.)