how to know where my phone is originally purchase

If you didn't buy the phone originally, I don't think there's any way to know where it was purchased at the beginning. You can check this link to see by model number where the phone may have been manufactured for, but that doesn't mean it was necessarily purchased where it was manufactured for --> Identify your iPhone model - Apple Support

If you aren't the original owner, if you know them, ask them where they bought the phone.

lobsterghost is correct; there is no way absent a receipt to know specifically where any Apple device was originally purchased. All you can do is determine the country for which the iPhone was intended.

If I may ask, why do you need to know? What issue are you facing?