How to identify if a PHAsset is not completely downloaded from iCloud (so I need to request again with options.networkAccessAllowed)

If you call requestImageDataForAsset: with networkAccessAllowed set to NO, the returned imageData will be nil if the clip is not already downloaded from iCloud. Otherwise it will return the actual data, even though the clip is stored in iCloud.

PHImageManager *manager = [PHImageManager defaultManager];    
PHImageRequestOptions *options = [[PHImageRequestOptions alloc] init];
options.networkAccessAllowed = NO;
[manager
 requestImageDataForAsset:asset
 options:options
 resultHandler:^(NSData * _Nullable imageData, NSString * _Nullable dataUTI, UIImageOrientation orientation, NSDictionary * _Nullable info) {
    if ([[info valueForKey:PHImageResultIsInCloudKey] boolValue]) {
    // Image is in iCloud
        if (imageData) {
            // Image is downloaded
        } else {
            // Image is not downloaded
        }
    }
 }];

I can confirm that PHImageResultIsInCloudKey is not reliable. For images stored in iCloud it returns 1, even though the original image has been downloaded to the device. This behavior is in contrast to the documentation and I would suggest to report a bug at radar.apple.com. PhotoKit in my opinion is still a very immature framework - it contains lots of issues and also some weird conceptional decisions.

You can use the progressHandler to check whether the PHAsset is from iCloud.

__block BOOL isPhotoInICloud = NO;
PHImageRequestOptions *options = [[PHImageRequestOptions alloc] init];
options.progressHandler = ^(double progress, NSError *error, BOOL *stop, NSDictionary *info){

    isPhotoInICloud = YES;
    // some code to update the download progress
});
options.networkAccessAllowed = YES;
options.deliveryMode = PHImageRequestOptionsDeliveryModeHighQualityFormat;
options.synchronous = NO;

[[PHImageManager defaultManager] requestImageForAsset:asset targetSize:PHImageManagerMaximumSize contentMode:PHImageContentModeAspectFit options:options resultHandler:^(UIImage * _Nullable result, NSDictionary * _Nullable info) {
// use the options to get the high quality image for only once time.
});