how to check befor buying i phone it is activated or inactivated

See >>> https://support.apple.com/en-us/HT201365

Do you mean Activation Lock as LACAllen interprets, or do you mean is it activated for service with a carrier and needs unlocking for use with a different carrier?

How to unlock your iPhone - http://support.apple.com/HT201328 - Only some carriers offer the ability to unlock iPhone for use on any carrier. Follow the instructions in this article to unlock your iPhone.