How do I know how long an iphone has been used before I bought it from someone else?

You cannot. There is no method to retrieve that information using the IMEI. If the phone is still activated, you could be able to see the Lifetime call time if you go to Settings>Cellular, but that will only provide you with total phone call usage statistics. There is nothing that indicates when the device was first activated, except to check the warranty expiration date, which requires the serial number of the device. You can then check the warranty status here, Check Your Service and Support Coverage - Apple Support The iPhone has a 1-year limited warranty, and the phone would have been activated 1-year back from the warranty expiration date.

Another method would be to ask to see the original purchase receipt.