In C#, how can I detect if a character is a non-ASCII character?

bool HasNonASCIIChars(string str)
{
    return (System.Text.Encoding.UTF8.GetByteCount(str) != str.Length);
}

ASCII ranges from 0 - 127, so just check for that range:

char c = 'a';//or whatever char you have
bool isAscii = c < 128;