An iOS engineer needs to be well versed in practically every major aspect of iOS development. In particular, depending on what your app does, do a little research on some core technologies that'll likely be involved. Are you a news app? Then someone with particular expertise with Core Text is probably essential.
Do you work with a lot of media (streaming video, encoding, etc.)? Then someone who knows AV Foundation is what you need. Games? OpenGL is a must. Apple has a lot of good, high-level guides that talk about all the technologies available to developers for particular tasks (audio, graphics, networking, etc.).
You can use these as a basis for what specializations you should look for in a senior iOS developer. You can also browse to sites like appdesign.ie/, to find an iOS developer.
Here are some of the absolute essentials that you'll definitely need someone to have a few years of solid experience with:
- Core Foundation
- Core Animation
- Core Graphics
- Core Data
If your app is particularly rich with transitions and animations, Core Animation intimacy is a requirement. If the candidate has a taste for design and has previous experience working closely with UI designers, then that's a really good sign.
Ask to see what apps they've built. Ask them what particular parts of apps they were responsible for. Ask them how they decide whether to use a third-party library or roll their own implementation.