It's really a judgement call by whoever is evaluating the device. Supposedly catastrophic damage isn't covered, but I believe that's usually as a response to people who intentionally damage their phones looking for a new/refurb at the out of warranty price. There are obviously cases where Apple has replaced an iPhone under the out of warranty program, but where it's clearly not repairable in any way from things such as water submersion or extreme drops.
You can just bring it in and ask. If you have access to an actually Apple (retail) Store then that would be the preferred place to go for an evaluation. Apple authorized service is a little different. They might charge a diagnostic fee that won't be refunded if your device isn't replaced for the fee.