meta box validation

In my custom post type i have a custom meta box field, so I want a database validation before saving my post and custom metabox data, .An example would give a better idea – Like what we have in Facebook registration form when we submit form there should be a database check for if given email id already exists on database or not, like this when i publish my custom post type post there should be a database check on the data i insert through custom meta box inside the custom post type. I have a FILE NO field in my custom post type, that i dont want another post with same FILE NO.
I am not using my functions.php file not a plugin