Have you tried using google search?
c# check if file exists - Google Search[
^] - there are a tonne of examples of how to do what you want...
theChkBox.IsChecked = System.IO.File.Exists(theTxtBox.Text);
** UPDATE **
How to do it automatically...
private void theTextBox_TextChanged(object sender, EventArgs e)
{
theCheckBox.Checked = System.IO.File.Exists(theTextBox.Text);
}