// do not forget to reference System.Text.RegularExpressions 
// This method will help you to detect if the string contains non English characters

public bool IsEnglish(string inputstring)
{
    var regex = new Regex(@"[A-Za-z0-9 .,-=+(){}\[\]\\]");
    var matches = regex.Matches(inputstring);
 
    return matches.Count.Equals(inputstring.Length);
}