Of course I can use client side javascript, but any server side solution for checkbox validation in webform? may be custom validator but not sure how to use it. Thanks in advance.

what kind of validation do you want to do?

whether checkbox is checked or not? (for eg. I agree your term and condition)

Check if the checkbox is checked:

ex:
In you code (what ever event.. maybe on click of a submit button)
        If CheckBox1.Checked Then
         do something ..
        Else
         do something else...
        End If