This filter provides facility to validate all fields of front side form (user meta details) before the form will get submitted.

Hook Type: Filter
Source: core/classes/class.arm_member_forms.php
add_filter( 'arm_validate_field_value_before_form_submission', 'your_function', 10, 3);
function your_function($return, $form, $posted_data) {
    //$return True or error message array if any.
    //$form Form Object.
    //$posted_data Posted form details.
    if($posted_data['some_key'] == '') {
        if(!is_array($return)) { $return = array(); }
        $return['some_key'] = 'Error message goes here';
    return $return;
True or error message array if any
Form Object
Posted form details