How do I verify a file's existence in iCloud?

To find files in iCloud, you use NSMetadataQuery. That will find both files that have already been downloaded as well as files that are in the user's account but which haven't been downloaded to the local device yet. Using NSFileManager will, at best, only find files that have already been downloaded.

You set it up with something like this:

NSMetadataQuery *query = [[NSMetadataQuery alloc] init];
[self setMetadataQuery:query];
[query setSearchScopes:@[NSMetadataQueryUbiquitousDataScope]];
[query setPredicate:[NSPredicate predicateWithFormat:@"%K LIKE '*'", NSMetadataItemFSNameKey]];

You'll want to observe NSMetadataQueryDidStartGatheringNotification, NSMetadataQueryDidUpdateNotification, and probably NSMetadataQueryDidFinishGatheringNotification. Then start the query:

[query startQuery];

With that done, you'll get notifications as the query discovers iCloud files. The notifications will include instances of NSMetadataItem, which you can use to get information like file size, download status, etc.

Use a metadata query - here is some sample code

/*!  Creates and starts a metadata query for iCloud files

 */
- (void)createFileQuery {
    [_query stopQuery];

        if (_query) {
            [_query startQuery];
        }
        else {
            _query = [[NSMetadataQuery alloc] init];

            [_query setSearchScopes:[NSArray arrayWithObjects:NSMetadataQueryUbiquitousDocumentsScope, NSMetadataQueryUbiquitousDataScope, nil]];
            // NSString * str = [NSString stringWithFormat:@"*.%@",_fileExtension];
            NSString *str = @"*";
            [_query setPredicate:[NSPredicate predicateWithFormat:@"%K LIKE %@", NSMetadataItemFSNameKey, str]];

            NSNotificationCenter* notificationCenter = [NSNotificationCenter defaultCenter];
            [notificationCenter addObserver:self selector:@selector(fileListReceived) name:NSMetadataQueryDidFinishGatheringNotification object:_query];
            [notificationCenter addObserver:self selector:@selector(fileListReceived) name:NSMetadataQueryDidUpdateNotification object:_query];
            [_query startQuery];
        }

}

/*! Gets called by the metadata query any time files change.  We need to be able to flag files that
 we have created so as to not think it has been deleted from iCloud.

 */
- (void)fileListReceived {
    LOG(@"fileListReceived called.");

    NSArray* results = [[_query results] sortedArrayUsingComparator:^(NSMetadataItem* firstObject, NSMetadataItem* secondObject) {
        NSString* firstFileName = [firstObject valueForAttribute:NSMetadataItemFSNameKey];
        NSString* secondFileName = [secondObject valueForAttribute:NSMetadataItemFSNameKey];
        NSComparisonResult result = [firstFileName.pathExtension compare:secondFileName.pathExtension];
        return result == NSOrderedSame ? [firstFileName compare:secondFileName] : result;
    }];

    //FLOG(@" results of query are %@", results);
    for (NSMetadataItem* result in results) {
        NSURL* fileURL = [result valueForAttribute:NSMetadataItemURLKey];
        NSString* fileName = [result valueForAttribute:NSMetadataItemDisplayNameKey];
        NSNumber* percentDownloaded = [result valueForAttribute:NSMetadataUbiquitousItemPercentDownloadedKey];
        NSNumber *isDownloaded = nil;
        NSNumber *isDownloading = nil;
        NSError *er;
        [fileURL getResourceValue: &isDownloaded forKey:NSURLUbiquitousItemIsDownloadedKey error:&er];
        [fileURL getResourceValue: &isDownloading forKey:NSURLUbiquitousItemIsDownloadingKey error:&er];
        FLOG(@" Found file %@", fileName);
   }

}

From the docs:

In iOS, actively download files when required. Items in iCloud but not yet local are not automatically downloaded by iOS; only their metadata is automatically downloaded. The initial download of new iCloud-based documents requires your attention and careful design in your app. After you explicitly download such an item, the system takes care of downloading changes arriving from iCloud.

Consider keeping track of file download status as part of your iOS app’s model layer. Having this information lets you provide a better user experience: you can design your app to not surprise users with long delays when they want to open a document that is not yet local. For each file (or file package) URL provided by your app’s metadata query, get the value of the NSURLUbiquitousItemIsDownloadedKeykey by calling the NSURL method getResourceValue:forKey:error:. Reading a file that has not been downloaded can take a long time, because the coordinated read operation blocks until the file finishes downloading (or fails to download).

For a file (or file package) that is not yet local, you can initiate download either when, or before, the user requests it. If your app’s user files are not large or great in number, one strategy to consider is to actively download all the files indicated by your metadata query. For each file (or file package) URL provided by the query, make the corresponding item local by calling the NSFileManager method startDownloadingUbiquitousItemAtURL:error:. If you pass this method a URL for an item that is already local, the method performs no work and returns YES.

Update: iOS7 should use NSURLUbiquitousItemDownloadingStatusKey instead of NSURLUbiquitousItemIsDownloadedKey.