How to implement extra validation

For node based content the easy way is to use the nodeapi hook and the case where $op is 'validate', The following is a working example, under your modules directory add a directory call 'node_validator'. Create the two files below (with the content show). The module file includes the nodeapi hook and once you enable the module every time you save content it will display the node type and the node (these are for debugging). The example validation code works on a content type called 'junk' with a text field called 'value'.

File node_validator.info

name = Node Validation
description = Skeleton for adding addition node validation

File node_validator.module

<?php
function node_validator_nodeapi(&$node, $op, $a3 = NULL, $a4 = NULL) {
	if ( $op == 'validate' ) {
		// The next two lines are meant to you find the information you want to validate
		drupal_set_message("Validating node type $node->type");
		drupal_set_message("<pre>" . print_r($node, TRUE) . '<pre>');
		
		// Example code for validation (focus on CCK generated types)
		// First is this a content type we care about?
		switch ( $node->type ) {
		case 'junk':
			// Check the value is validate
			// Here you need to now how to access the value(s)
			// Here since we are using CCK we allow for multiple values
			
			// For example we are going to make sure the value field
			// is in the range 20 to 200 (including 20 and 200
			
			foreach ( (array)$node->field_value as $instance ) {
				// Make sure the value is actually set
				if ( isset($instance['value']) ) {
					$value = $instance['value'];
					if ( is_numeric($value) ) {
						// It's a number, is it in range?
						if ( $value < 20 || $value > 200 ) {
							form_set_error('field_type', "Value must be a number in the range 20 to 200");
						}
					}
					else {
						// Not a number, generate an error
						form_set_error('field_type', "Value must be a number in the range 20 to 200");
					}
				}
			}
			break;
		}
	}
}

Also read this:
http://drupal.org/node/134289