How Do I Validate a textbox (make sure it's not empty) when a checkbox is checked

I have been looking for several hours trying to find code for requiring a customer to enter qty in the qty_field when a checkbox is checked.  Here's what I am trying to do:

i have several test available, so when customers checked on Checkbox1(or Checkbox2); they must also enter a qty for the qty_field.  The qty_field is used for calculation later.  I try ASP.net 2.0 Validation, but it does not have option for me to tell it that if checkbox1 is checked, then qty1 must also cannot be empty. I am storing my values using session.  what i have below does not work (and no error message), just no result.  

How do I validate the checkbox and the textbox before submitting the form?

thanks


my aspx code:
<form id="form2" runat="server">
<div>
<table cellpadding="4" cellspacing="0" width="100%" border="1">
         <tr>
       <td width="40%">
      <asp:CheckBox ID="CheckBoxLT1" Text="Test1" runat="server" />
          </td>
      <td width="10%">
      <asp:TextBox ID="txtltNumTest" runat="server" Width="70px"></asp:TextBox>
    </td>
</tr>
<tr> <td width="40%"><asp:CheckBox ID="CheckBoxLT2" Text="Test2" runat="server" />
 </td>   <td width="10%"><asp:TextBox ID="txtltNumTest2" runat="server" style="height: 22px"
 Width="70px"></asp:TextBox>
</td> </tr>
</table>
<asp:Button ID="Button1" runat="server" Text="Submit" />
</div>
</form>


aspx.vb code:

Protected Sub Button1_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles Button1.Click

 'check to see which test is checked
       'check to see which test is checked
        If CheckBox1.Checked = True Then
            If AntiNumTest.Text <> "" Then
                Session("testqty") = CInt(AntiNumTest.Text)
                Session("anticost") = Session("testqty") * 250
            Else
                response.write ("Please Enter qty for test")
            End If
        End If

Session("Cost") = Session("anticost")
Response.Redirect("page2.aspx")

End Sub