I have written an app and its base SDK is set to 4.2, and at the moment it is in the app store with a deployment target set to 4.2, I’m sure you’ll agree this is a bad idea because I am alienating all potential users who do not have the latest version of iOS installed.
So my question is, when deploying for an older version of iOS, how do I make sure it will run, i.e. how do I check whether I’ve used any modern API’s that are not present in the deployment target iOS version?
The only sure-fire way to make sure your app will run is to test on a device running the older OS version you specify.
Even old versions of the iPhone Simulator (if you can find such) will not do a good job of checking for compatibility.
Most developers who want to support customers on iPhoneOS 3.x have access to an old test device, such as an used iPod Touch (hand-me-down, or purchased on eBay, etc.) running the OS version set in the app’s Deployment Target. Otherwise, the developer is just guessing/gambling.