更新するだけで、これは可能です。
device = [UIDevice currentDevice];
// Turn on proximity monitoring
[device setProximityMonitoringEnabled:YES];
// To determine if proximity monitoring is available, attempt to enable it.
// If the value of the proximityMonitoringEnabled property remains NO, proximity
// monitoring is not available.
// Detect whether device supports proximity monitoring
proxySupported = [device isProximityMonitoringEnabled];
// Register for proximity notifications
[notificationCenter addObserver:self selector:@selector(proximityChanged:) name:UIDeviceProximityStateDidChangeNotification object:device];
benzado が指摘しているように、以下を使用できます。
// Returns a BOOL, YES if device is proximate
[device proximityState];