Here i am explaining How to call Magento Varien Form Validation on Button click instead of form submit,
Some time you need to create same form in two steps like Step 1 with some fields and Step 2 with some fields, I this case before going to Step 2 you need to validate Step 1 form fields.
Here i am explaining how it can be done.
var formToValidate = $('form-validate');
var validator = new Validation(formToValidate);
// Here is your code for showing Next step or Do whatever you want.