Select the Enum to classify a string based on the first word found
You can utilise the Enum class which can shorten your code significantly and it will be easier for future updates

private static QuestionIdentifier IdentifyQuestion(string vQuestion)
{
    //QuestionToLower
    string vQTL = vQuestion.ToLower();
    string[] enumNames = Enum.GetNames(typeof(QuestionIdentifier));
    foreach (var enumName in enumNames)
    {
        if (vQTL.Contains(enumName.ToLower()))
        {
            return (QuestionIdentifier) Enum.Parse(typeof(QuestionIdentifier), enumName);
        }
    }
    return QuestionIdentifier.Andere;
}


or with LINQ

private static QuestionIdentifier IdentifyQuestion(string vQuestion)
{
    string vQTL = vQuestion.ToLower();
    string[] enumNames = Enum.GetNames(typeof(QuestionIdentifier));
    string enumValueName = enumNames.FirstOrDefault(x =&gt; vQTL.Contains(x.ToLower()));
    return enumValueName != null
        ? (QuestionIdentifier) Enum.Parse(typeof(QuestionIdentifier), enumValueName)
        : QuestionIdentifier.Andere;
}


Now you can add as much types as you want in your enum without even touching this method it will still work and find your newly created type. 

Update

As @t3chb0t pointed in the comments there is a way to avoid calling .ToLower() on both the input string and the enum value by using the .IndexOf() overload which accepts different comparison types :

private static QuestionIdentifier IdentifyQuestion(string vQuestion)
{
    string[] enumNames = Enum.GetNames(typeof(QuestionIdentifier));
    string enumValueName =
        enumNames.FirstOrDefault(x =&gt; vQuestion.IndexOf(x, StringComparison.OrdinalIgnoreCase) &gt;= 0);
    return enumValueName != null
        ? (QuestionIdentifier)Enum.Parse(typeof(QuestionIdentifier), enumValueName)
        : QuestionIdentifier.Andere;
}