I want to buy an iphone 12 from someone . How can I know that if this mobile is unlocked and not used before?

This is a user to user support forum, so nobody can check anything for you. And as I said, there is no way to check it. The one and the only way to know if it is activation locked would be to power it on and see if it can be setup with your AppleID. There is no remote or pre-check for activation lock. Activation lock has nothing to do with serial numbers nor IMEI numbers.

Without having the device physically in your hands to check it, there is no way to know for certain it is carrier unlocked and also that it is not activation locked. The only way to check if neither of those is in effect is to have that device in your hands to check it.