Вот фрагмент кода, который вы можете использовать:
[[Reachability sharedReachability] setHostName:@"example.com"];
if ([[Reachability sharedReachability] remoteHostStatus] == NotReachable) {
UIAlertView *dialog = [[[UIAlertView alloc] init] retain];
[dialog setTitle:@"Server unreachable"];
[dialog setMessage:@"The server is temporarily unavailable."];
[dialog addButtonWithTitle:@"OK"];
[dialog show];
[dialog release];
}