SDWebImage checking if image cached using Swift: Ambiguous Reference

cachedImageExistsForURL is not a class method on SDWebImageManager, it's an instance method which you need to use on the sharedInstance:

SDWebImageManager.sharedManager().cachedImageExistsForURL(imgURL)

As of SDWebImage 5 there is a new class, SDImageCache, which provides a synchronous method to use:

SDImageCache.shared.diskImageDataExists(withKey: urlString)

There is also an asynchronous method with a completion handler:

SDImageCache.shared.diskImageExists(withKey: urlString) { exists in
    // your code here
}