How do you know if you're eligible to upgrade to a new iPhone?

From your old phone, just dial *NEW# (*639#) or send a blank text message to the aforementioned number. You will receive a text message. The message will contain either:
  1. Something along the lines of "You are eligible for an early upgrade, just pay a $18 upgrade fee", or:
  2. An apology, telling you you can't upgrade and listing when you become eligible.