Możesz użyć np.: String.Contains Method
using System;
using System.Diagnostics;
using System.Text;
using System.Windows.Forms;
namespace WindowsFormsApplication1
{
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}
private void textBox1_TextChanged(object sender, EventArgs e)
{
string searchingWord = "hello";
if (textBox1.Text.Contains(searchingWord))
{
Debug.WriteLine("Znalazłem słowo " + searchingWord);
MessageBox.Show("Znalazłem słowo " + searchingWord);
}
}
}
}

lub String.IndexOf Method
using System;
using System.Diagnostics;
using System.Text;
using System.Windows.Forms;
namespace WindowsFormsApplication1
{
public partial class Form1 : Form
{
private static int indexOfSearchingWord = 0;
private static int lastIndexOfSearchingWord = 0;
public Form1()
{
InitializeComponent();
label1.Text = "";
}
private void textBox1_TextChanged(object sender, EventArgs e)
{
string searchingWord = "hello";
indexOfSearchingWord = textBox1.Text.IndexOf(searchingWord, lastIndexOfSearchingWord);
if (lastIndexOfSearchingWord <= indexOfSearchingWord)
{
lastIndexOfSearchingWord = indexOfSearchingWord + 1;
label1.Text += "Znalazłem słowo " + searchingWord + " na pozycji " + indexOfSearchingWord + "\n";
Debug.WriteLine("Znalazłem słowo " + searchingWord);
MessageBox.Show("Znalazłem słowo " + searchingWord);
}
}
}
}
