Magento – Validate input filed without form

I really need to have custom or even magento validation for my input field.

Problem is, there is no form in my field, there is only input field.

<input id="item_custom_payment_" name="pay_part_payment" value="" style="" class="input-text item-price validate-greater-than-zero">
<button .... >Submit</button> //calling custom controller action

For example: Comment section in order view page in admin area.

This var customForm = new VarienForm('custom_form'); doesn't work because there is no <form> in my section.

Is there anyway around to validate the input field?

UPDATE

My button looks like this:

<button id="id_5bb6f3992cb320bf703ed93f702dd64a" title="Submit Paid Amount" type="button" class="scalable save paid-amount" onclick="submitAndReloadArea($('order_paid_block_wrap').parentNode, '[admin_url]'); javascript:reload(); return true" style=""><span><span><span>Submit Paid Amount</span></span></span></button>