Check whether a video file is interlace/progressive

I am trying to see whether a video file the user has selected is interlaced/progressive, and then do some manipulation depending on that.

I have tried to check whether the cmsamplebuffer i have extracted is defined as top field first or bottom field first however this returns null for all inputs.

NSMutableDictionary *pixBuffAttributes = [[NSMutableDictionary alloc] init];
[pixBuffAttributes setObject:
[NSNumber numberWithInt:kCVPixelFormatType_422YpCbCr8]
forKey:(NSString*)kCVPixelBufferPixelFormatTypeKey];
myAsset = [[AVURLAsset alloc] initWithURL:urlpath options:pixBuffAttributes];
myAssetReader = [[AVAssetReader alloc] initWithAsset:myAsset error:nil];
myAssetOutput = [[AVAssetReaderTrackOutput alloc]initWithTrack:
  [[myAsset tracksWithMediaType:AVMediaTypeVideo]
  objectAtIndex: 0]
  outputSettings:pixBuffAttributes];
[myAssetReader addOutput:myAssetOutput];
[myAssetReader startReading];
CMSampleBufferRef ref = [myAssetOutput copyNextSampleBuffer];
if(CVBufferGetAttachments(ref, kCVImageBufferFieldDetailKey, nil) == nil)
{
  //always the case
}
else
{
  //never happens
}

regardless of the input file's interlacing the above always returns nil. I'm probably trying to test this in totally the wrong manor, so any help much appreciated!