How can I see the last location of an iphone that is already offline?

If you have find my enabled, you can see the last known location of the device in the Find My app.

Log in to https://icloud.com/find and chose the phone from the devices list. If the last location is available it will be displayed, but it may or may not be available, depending on whether you had record last location on, and also how long it has been since the phone was last “seen”.