Fastest way to check if an object exists in Core Data or not?

Setup a Core Data request and, instead of actually issuing the query, do the following:

NSError *error = nil;
NSUInteger count = [managedObjectContext countForFetchRequest:request
                                                        error:&error];
if (!error) {
    return count;
} else {
  return 0;
}

In practice, the method countForFetchRequest:error: returns the number of objects a given fetch request would have returned if it had been passed to executeFetchRequest:error:.

Edit: ^{(by Regexident)}

As Josh Caswell correctly commented, the correct way to handle errors is either this:

if (count == NSNotFound) {
    NSLog(@"Error: %@", error);
    return 0;
}
return count;

or this (without error logging):

return (count != NSNotFound) ? count : 0;