Check if exactly none, exactly one, or many extension method from http://stackoverflow.com/a/6940337/34315 and http://stackoverflow.com/a/6059711/34315.

Multiplicity.cs

public static Multiplicity Multiplicity<TElement>(this IEnumerable<TElement> @this)
{
    switch (@this.Take(2).Count())
    {
        case 0: return General.Multiplicity.None;
        case 1: return General.Multiplicity.One;
        case 2: return General.Multiplicity.Many;
        default: throw new Exception("WTF‽");
    }
}

public enum Multiplicity
{
    None,
    One,
    Many,
}