The simplest solution is to use regular expressions and the word boundary delimiter \b:
bool result = Regex.IsMatch(text, "\\bthe\\b");
or, if you want to find mismatching capitalisation,
bool result = Regex.IsMatch(text, "\\bthe\\b", RegexOptions.IgnoreCase);
(using System.Text.RegularExpressons.)
Alternatively, you can split your text into individual words and search the resulting array. However, this isn’t always trivial because it’s not enough to split on white spaces; this would ignore all punctuation and yield wrong results. A solution is to once again use regular expressions, namely Regex.Split.