How to find an iPhone when it's offline?

That isn't possible.

If Your iDevice Is Lost Or Stolen

If you activated Find My Phone before it was lost or stolen, you can track it only when it is connected to the Internet by Wi-Fi or cellular. What you cannot do is track your device using a serial number or other identifying numbers. Neither Apple nor anyone else can find your device. You cannot recover your loss unless you ensure your device for such loss. It is not covered by your warranty.

If your iPhone, iPod, iPod Touch, or iPad is lost or stolen what do you do? There are things you should have done in advance - before you lost it or it was stolen - and some things to do after the fact. Here are some suggestions:

  1. Reporting a lost or stolen Apple product

  2. Find my lost iPod Touch

  3. AT&T. Sprint, and Verizon can block stolen phones/tablets

  4. What-To-Do-When-Iphone-Is-Stolen

  5. iCloud- Use Lost Mode

  6. What to do if your iOS device is lost or stolen

  7. 6 Ways to Track and Recover Your Lost/Stolen iPhone

  8. Set up Find My on your iPhone, Mac, and other devices - Apple Support

  9. Report Stolen iPad | Stolen Lost Found Online

It pays to be proactive by following the advice on using Find My Phone before you lose your device:

  1. Set up Find My on your iPhone, Mac, and other devices - Apple Support

  2. Setup your iDevice on iCloud

  3. OS X Lion/Mountain Lion- About Find My Mac

  4. How To Set Up Free Find Your iPhone (Even on Unsupported Devices)

You can't, except by doing a manual search. Find My Phone requires the device to be on line.