jquery check if asp checkbox is checked

Assuming your checkbox is the only item on the page having the checkbox class:

var checked = $(".checkbox").is(':checked')

try...

if ($('#<%= checkboxRules.ClientID %>').is(':checked')) {
...
}

Since it is a server-side Checkbox, it will send something like <input type="checkbox" class="checkbox" /> as HTML to the client after ASP.NET processes the control.

The id of the checkbox isn't going to be checkboxRules as you have it in the source code. ASP.NET will make a concatenation of the server-side form id + master page id (if using a master page) + checkboxRules so in this case I won't use a selector that depends on element id.

We can make a jQuery selector as narrow as possible to only select inputs with a type of "checkbox" and with a CSS class of "checkbox".

$('input[type=checkbox] .checkbox').attr('checked')

will return the boolean value of the check status of the input. This will find any input on the page that is a checkbox with that CSS class.