The IList<T> interface has an IndexOf method (and a corresponding Item property). The IEnumerable<T> interface does not have an equivalent IndexOf method (though it does have an extension method ElementAt<T> that mirrors the Item property).
Naturally, you can add your own extension method:
public static int IndexOf<TSource>(this IEnumerable<TSource> source, TSource t) { var index = source.TakeWhile(s => s != t).Count(); return index == source.Count() ? -1 : index; }