ASP.net ships with a couple of validator controls that allow you to determine whether the value of the input controls they are validating is valid.
Here is a simple example of a TextBox control with a RequiredFieldValidator attached and a Button control.

<asp:TextBox ID="TextBox1" runat="server" ValidationGroup="MyValidationGroup"></asp:TextBox>

<asp:RequiredFieldValidator ID="RequiredFieldValidator1" runat="server" ControlToValidate="TextBox1"

 ErrorMessage="This field is required!" ValidationGroup="MyValidationGroup"></asp:RequiredFieldValidator>

<br />

<asp:Button ID="Button1" runat="server" OnClick="Button1_Click" Text="Button" ValidationGroup="MyValidationGroup" />

With JavaScript turned off, what may not be known is that, on the server side, even though the validators fire, it is left to the developer on how to use that information.  

You may think you have built a secure application but a hacker could disable JavaScript and bypass *all* your validators! This is where the Page.Validate method and more importantly, the Page.IsValid property come in.
The Validate method is fired automatically by controls that have the CausesValidation property set to true. Note that the Button control's CausesValidation property is true by default. The Validate method iterates through all enabled validation controls on the page and validates them. This event occurs after the Load event in the page lifecycle.
If the control that raised the event has a ValidationGroup specified, then only enabled validator controls that belong to the same ValidationGroup are validated by calling the Page.Validate(ValidationGroup) overload method. As mentioned previously, this is done automatically for controls that have the CausesValidation property set to true.
The Page.IsValid property tells you whether the validation succeeded or not. It can be called only after the Page.Validiate method is called. By using this property, you can add logic to your page to determine whether to proceed with the PostBack event or not. So, in addition to relying on client side validation, it is also important that you call Page.IsValid when handling the postback event. Here is what the code behind will look like:

protected void Button1_Click(object sender, EventArgs e) {

//Not required since the CausesValidation property of the Button control is true by default.

//Page.Validate("MyValidationGroup");

//Proceed only if the vaidation is successfull

if (!Page.IsValid) {

 return;

}

//Insert data into SQL

}