How does one tell if a LINQ extension method is suffering from double enumeration?

Question

I am wondering what rules there are to tell if a portion of LINQ code is suffering from double enumeration?

For example, what are the telltale signs that the following code might be double enumerating? What are some other signs to look out for?

public static bool MyIsIncreasingMonotonicallyBy<T, TResult>(this IEnumerable<T> list, Func<T, TResult> selector)
    where TResult : IComparable<TResult>
{
    return list.Zip(list.Skip(1), (a, b) => selector(a).CompareTo(selector(b)) <= 0).All(b     => b);
}

Answer 1

Pass in one of these:

public class OneTimeEnumerable<T> : IEnumerable<T>
{
  public OneTimeEnumerable(IEnumerable<T> source)
  {
    _source = source; 
  }

  private IEnumerable<T> _source;
  private bool _wasEnumerated = false;

  public IEnumerator<T> GetEnumerator()
  {
    if (_wasEnumerated)
    {
      throw new Exception("double enumeration occurred");
    }
    _wasEnumerated = true;
    return _source.GetEnumerator();
  }

}