test.cs

[Test]
public static void Class_AllStaticMethodsShouldBeDecoratedWithDebuggerStepThroughAttribute()
{
    var methods = StaticMethodsWithout(typeof(Mooble.Util.StringExtensions), typeof(DebuggerStepThroughAttribute))
    methods.Count.Should.Be(0);
}

IEnumerable<MethodInfo> StaticMethodsWithout(Type type, Type attribute)
{
 return from m in type.GetMethods()
                    where !m.IsSpecialName && m.IsStatic &&
                        m.GetCustomAttributes(attribute, true).Count() == 0
                    select m.Name;
}

Esta ok