Marcus Zarra has a great post called "Five Things To Ask Your (Potential) iOS Developer", well worth a read.
And I've got 5 more questions to suggest:
- Can you provide the last 20 check-in comments for a recent project?
- What is your preferred code formatting style?
- How, and how often, do you distribute ad-hoc builds?
- How do you gather and analyze usage data and crash reports?
- What is the session failure rate for your apps?
Can you provide the last 20 check-in comments for a recent project?
No need to see the sources, just the comments. A good developer will be very specific with source control check-ins, grouping related files together with thoughtful comments. A rookie developer will check in a bunch of different changes with comments like "bug fixes" or "changes". Also, a good developer may be pushing several check-ins per development day.
What is your preferred code formatting style?
There are actually a variety of named code formatting styles, see http://en.wikipedia.org/wiki/Indent_style. My personal favorite is Allman style.
The real question is, how consistently does your (potential) iOS developer adhere to their chosen style in their own code? Obviously, style in-and-of-itself doesn't affect how well the code can work, but it does give you insight into the discipline of the engineer. Sloppy code is a red flag.
How, and how often, do you distribute ad-hoc builds?
If they aren't using HockeyApp or TestFlight, ask them why not? And if they aren't generating ad-hoc builds weekly, if not daily, ask them why not? A good developer is constantly practicing the process of building apps ready for release.
How do you gather and analyze usage data and crash reports?
Jeremy Fuller has a great post called "The battle of the iOS crash reporters", and your candidate should be using one of these. And they should also be using analytics such as Flurry to understand new users, active users, session lengths, usage paths, etc.
What is the session failure rate for your apps?
This is probably the most important question, and should, in fact, be your first question! A session failure to me means the app crashed and so SFR = number of crashes / the number of sessions.
Per a Forbes article from last year, iOS itself has an overall session failure rate of ~3%. So if an app has a session failure rate close to 3%, its on par with the very best apps available, including Apple's. IMHO, an ~8% SFR is rather high, and anything > 10% is simply unacceptable.