gistfile1.m

//
// OLD AND BUSTED
//

if ([self.zoomingDelegate respondsToSelector:@selector(zoomingWindow:didZoomOutViewController:)] == YES)
{
// Do something important.
}

//
// NEW HOTNESS
//

// 1. A private struct with an option for each method you want to check against.

struct {
    unsigned willZoomInViewController : 1;
    unsigned didZoomInViewController : 1;
    unsigned willZoomOutViewController : 1;
    unsigned didZoomOutViewController : 1;
} _delegateHas;


// 2. Set those values in the delegate's setter.
- (void)setZoomingDelegate:(id<SGZoomingWindowDelegate>)zoomingDelegate
{
    _zoomingDelegate = zoomingDelegate;
    _delegateHas.willZoomInViewController = [_zoomingDelegate respondsToSelector:@selector(zoomingWindow:willZoomInViewController:)];
    _delegateHas.didZoomInViewController = [_zoomingDelegate respondsToSelector:@selector(zoomingWindow:didZoomInViewController:)];
    _delegateHas.willZoomOutViewController = [_zoomingDelegate respondsToSelector:@selector(zoomingWindow:willZoomOutViewController:)];
    _delegateHas.didZoomOutViewController = [_zoomingDelegate respondsToSelector:@selector(zoomingWindow:didZoomOutViewController:)];
}

// 3. Call this shorter, easier to read snippet of code in-line

if (_delegateHas.didZoomOutViewController)
{
// Do stuff
}

Nice optimization.