First, check if the device is running on iOS 8.The easiest and best way is to call MTLCreateSystemDefaultDevice which will check if the Metal API is available.
If you get a valid object is returned, then the device is an A7 or an A8 chip, and, therefore, Metal is available. Otherwise, the this is older device, and it is required to use something like an OpenGL ES 2.0 context for a fast graphics pipeline.
More information with Apple’s Metal API Guide.