Are headphones plugged in? iOS7

This should work, but I cannot test it right now, I'll do in the evening.

- (BOOL)isHeadsetPluggedIn {
    AVAudioSessionRouteDescription* route = [[AVAudioSession sharedInstance] currentRoute];
    for (AVAudioSessionPortDescription* desc in [route outputs]) {
        if ([[desc portType] isEqualToString:AVAudioSessionPortHeadphones])
            return YES;
    }
    return NO;
}

Just to extend @Antonio's answer. If you need to detect whether the user has pulled out or plugged in the headphone.

#import <AVFoundation/AVFoundation.h>

// [AVAudioSession sharedInstance]; // @Boris edited: you may need it if there is no `AVAudioSession instance` created before. If doesn't work, uncomment this line.
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(audioRouteChangeListenerCallback:)
                                             name:AVAudioSessionRouteChangeNotification
                                           object:nil];
// don't forget to `removeObserver:`

// If the user pulls out he headphone jack, stop playing.
- (void)audioRouteChangeListenerCallback:(NSNotification*)notification
{
    NSDictionary *interuptionDict = notification.userInfo;

    NSInteger routeChangeReason = [[interuptionDict valueForKey:AVAudioSessionRouteChangeReasonKey] integerValue];

    switch (routeChangeReason) {

        case AVAudioSessionRouteChangeReasonNewDeviceAvailable:
            NSLog(@"AVAudioSessionRouteChangeReasonNewDeviceAvailable");
            NSLog(@"Headphone/Line plugged in");
            break;

        case AVAudioSessionRouteChangeReasonOldDeviceUnavailable:
            NSLog(@"AVAudioSessionRouteChangeReasonOldDeviceUnavailable");
            NSLog(@"Headphone/Line was pulled. Stopping player....");
            break;

        case AVAudioSessionRouteChangeReasonCategoryChange:
            // called at start - also when other audio wants to play
            NSLog(@"AVAudioSessionRouteChangeReasonCategoryChange");
            break;
    }
}

Swift 3:

To check if headphones are connected

extension AVAudioSession {

    static var isHeadphonesConnected: Bool {
        return sharedInstance().isHeadphonesConnected
    }

    var isHeadphonesConnected: Bool {
        return !currentRoute.outputs.filter { $0.isHeadphones }.isEmpty
    }

}

extension AVAudioSessionPortDescription {
    var isHeadphones: Bool {
        return portType == AVAudioSessionPortHeadphones
    }
}

Then you can just print("isHeadphones connected: \(AVAudioSession.isHeadphonesConnected)")

Listening to Changes

In Swift 3 the syntax is this:

func handleRouteChange(_ notification: Notification) {
    guard
    let userInfo = notification.userInfo,
    let reasonRaw = userInfo[AVAudioSessionRouteChangeReasonKey] as? NSNumber,
    let reason = AVAudioSessionRouteChangeReason(rawValue: reasonRaw.uintValue)
    else { fatalError("Strange... could not get routeChange") }
    switch reason {
    case .oldDeviceUnavailable:
        print("oldDeviceUnavailable")
    case .newDeviceAvailable:
        print("newDeviceAvailable") 
        if AVAudioSession.isHeadphonesConnected {
             print("Just connected headphones")
        } 
    case .routeConfigurationChange:
        print("routeConfigurationChange")
    case .categoryChange:
        print("categoryChange")
    default:
        print("not handling reason")
    }
}

func listenForNotifications() {
    NotificationCenter.default.addObserver(self, selector: #selector(handleRouteChange(_:)), name: NSNotification.Name.AVAudioSessionRouteChange, object: nil)
}

Notice use of:

 if AVAudioSession.isHeadphonesConnected {
    print("Just connected headphones")
 }

@Warif's code in Swift 2.0 with little changes ...

func audioRouteChangeListenerCallback (notif: NSNotification){
        let userInfo:[NSObject:AnyObject] = notif.userInfo!
        println("\(userInfo)")
        let routChangeReason = UInt((userInfo[AVAudioSessionRouteChangeReasonKey]?.integerValue)!)
        switch routChangeReason {
        case AVAudioSessionRouteChangeReason.NewDeviceAvailable.rawValue:
            self.println("Headphone/Line plugged in");
            break;

        case AVAudioSessionRouteChangeReason.OldDeviceUnavailable.rawValue:
            //If the headphones was pulled move to speaker
            do {
                try AVAudioSession.sharedInstance().overrideOutputAudioPort(AVAudioSessionPortOverride.Speaker)
            } catch _ {
            }
            self.println("Headphone/Line was pulled. Stopping player....");
            break;

        case AVAudioSessionRouteChangeReason.CategoryChange.rawValue:
            // called at start - also when other audio wants to play
            self.println("AVAudioSessionRouteChangeReasonCategoryChange");
            break;
        default:
            break;
        }
    }

:D

In Swift (as of 1.2):

    func headsetPluggedIn() -> Bool {
    let route = AVAudioSession.sharedInstance().currentRoute
    return (route.outputs as! [AVAudioSessionPortDescription]).filter({ $0.portType == AVAudioSessionPortHeadphones }).count > 0
}