Check if UserDefault exists - Swift

This is essentially the same as suggested in other answers but in a more convenient way (Swift 3+):

extension UserDefaults {
    static func contains(_ key: String) -> Bool {
        return UserDefaults.standard.object(forKey: key) != nil
    }
}

usage: if UserDefaults.contains(kUSERID) { ... }

I copy/pasted your code but Xcode 6.1.1 was throwing some errors my way, it ended up looking like this and it works like a charm. Thanks!

func userAlreadyExist(kUsernameKey: String) -> Bool {
    return NSUserDefaults.standardUserDefaults().objectForKey(kUsernameKey) != nil
}

Swift 5:

if UserDefaults.standard.object(forKey: "keyName") != nil {
  //Key exists
}

Astun has a great answer. See below for the Swift 3 version.

func isKeyPresentInUserDefaults(key: String) -> Bool {
    return UserDefaults.standard.object(forKey: key) != nil
}

Yes this is right way to check the optional have nil or any value objectForKey method returns AnyObject? which is Implicit optional.

So if userDefaults.objectForKey(kUSERID) have any value than it evaluates to true. if userDefaults.objectForKey(kUSERID) has nil value than it evaluates to false.

From swift programming guide

If Statements and Forced Unwrapping You can use an if statement to find out whether an optional contains a value. If an optional does have a value, it evaluates to true; if it has no value at all, it evaluates to false.

Now there is a bug in simulators than after setting key in userDefaults they always remain set no matter you delete your app.You need to reset simulator.

Reset your Simulator check this method before setting key in userDefaults or remove key userDefaults.removeObjectForKey(kUSERID) from userDefaults and it will return NO.On devices it is resolved in iOS8 beta4.