有两点要检查：URL 本身是否有效，以及服务器响应没有错误。

There's two things to check: if the URL itself is valid, and if the server responds without error.

在我的示例中，我使用的是HEAD请求，它避免了下载整个页面，并且几乎不占用带宽。

In my example I'm using a HEAD request, it avoids downloading the whole page and takes almost no bandwidth.

func verifyURL(urlPath: String, completion: (isValid: Bool)->()) {
    if let url = NSURL(string: urlPath) {
        let request = NSMutableURLRequest(URL: url)
        request.HTTPMethod = "HEAD"
        let task = NSURLSession.sharedSession().dataTaskWithRequest(request) { (_, response, error) in
            if let httpResponse = response as? NSHTTPURLResponse where error == nil && httpResponse.statusCode == 200 {
                completion(isValid: true)
            } else {
                completion(isValid: false)
            }
        }
        task.resume()
    } else {
        completion(isValid: false)
    }
}

用法：

verifyURL("http://google.com") { (isValid) in
    print(isValid)
}

用于游乐场，不要忘记启用异步模式以便能够使用NSURLSession：

For use in a Playground, don't forget to enable the asynchronous mode in order to be able to use NSURLSession:

import XCPlayground
XCPlaygroundPage.currentPage.needsIndefiniteExecution = true