A simple test if a mounted volume or device at a given path is a disk image (DMG).

SomeObject.m

#import <DiskArbitration/DiskArbitration.h>
// ...
- (BOOL)isDMGVolumeAtURL:(NSURL *)url
{
    
  BOOL isDMG = NO;
    
  if (url.isFileURL) {
    
    DASessionRef session = DASessionCreate(kCFAllocatorDefault);
    if (session != nil) {
      
      DADiskRef disk = DADiskCreateFromVolumePath(kCFAllocatorDefault, session, (__bridge CFURLRef)url);
      if (disk != nil) {
        
        NSDictionary * desc = CFBridgingRelease(DADiskCopyDescription(disk));
        NSString * model = desc[(NSString *)kDADiskDescriptionDeviceModelKey];
        isDMG = ([model isEqualToString:@"Disk Image"]);
        
        CFRelease(disk);
        
      }
      
      CFRelease(session);
      
    }
    
  }
  
  return isDMG;
  
}