If the asset is a file in local folder - no problem, you can use NSFileManager to check for existance? But what if it's inside the iPod library? The following trick wasn't easy to find, but here it is:
NSURL* furl = [f trackUrl];
// ipod-library://item/item.mp3?id=
if([[furl scheme] isEqualToString:@"ipod-library"]){
NSNumber* pid = [NSNumber numberWithLongLong: [[f.name substringFromIndex:32] longLongValue] ];;
MPMediaPropertyPredicate *predicate = [MPMediaPropertyPredicate predicateWithValue:pid forProperty:MPMediaItemPropertyPersistentID];
MPMediaQuery *songQuery = [[[MPMediaQuery alloc] init] autorelease];
[songQuery addFilterPredicate: predicate];
if (songQuery.items.count == 0) {
return NO; // NOT FOUND!
}