Check if a UIAlertView is showing

On the object that calls set an ivar before invoking the show method on your UIAlertView.

...

if (!self.alertShowing) {
    theAlert = [[UIAlertView alloc] initWithTitle:title message:details delegate:self cancelButtonTitle:nil otherButtonTitles:@"Okay", nil];
    self.alertShowing = YES;
    [theAlert show];
}

...

Then in your delegate method for the alert manage setting your flag ivar to no:

- (void)alertView:(UIAlertView *)alertView didDismissWithButtonIndex:(NSInteger)buttonIndex
{
  ...
      self.alertShowing = NO;
}

If you want the alerts to show sequentially, I would post notifications to add each message to a queue and then only take a message off the queue after an alert is dismissed.

If you can control the other alert views, check the visible property for each of them.

---

In iOS 6 or before, when an alert appears, it will be moved to a _UIAlertOverlayWindow. Therefore, a pretty fragile method is to iterate through all windows and check if there's any UIAlertView subviews.

for (UIWindow* window in [UIApplication sharedApplication].windows) {
  NSArray* subviews = window.subviews;
  if ([subviews count] > 0)
    if ([[subviews objectAtIndex:0] isKindOfClass:[UIAlertView class]])
      return YES;
}
return NO;

This is undocumented as it depends on internal view hierarchy, although Apple cannot complain about this. A more reliable but even more undocumented method is to check if [_UIAlertManager visibleAlert] is nil.

These methods can't check if a UIAlertView from SpringBoard is shown.

Why not just check the visible property, maintained by the UIAlertView class?

if (_alert) //alert is a retained property
{
    self.alert = [[[UIAlertView alloc] initWithTitle:@"Your Title"
                                             message:@"Your message" 
                                            delegate:self
                                   cancelButtonTitle:@"Cancel"
                                   otherButtonTitles:@"OK"] autorelease];
}
if (!_alert.visible)
{
    [_alert show];
}