verification of textbox

You can do this using the javascript on 'onblur' event of textbox.

Try something like this:

<asp:textbox id="your_id" runat="server" onblur="your script function" />

Use TextChanged event on TextBox

void textBox1_TextChanged(object sender, EventArgs e)
{
    //you code for verify
}

you should have write more detail about what you have to verify. for example I have verify the text with text length not less then 8 and not greater then 22. Create one textbox (TB) and one Lable (lB)
here the code:

private void textBox1_TextChanged(object sender, EventArgs e)
        {
            lB.Text = test(tb.Text);
        }
        public string test(string test){
            int a=test.Length;
            if (a > 8  && a<22)
            {
                return "Ok";
            }
            else
            {
                return "Not Valid";
            }
            }
        }