VALIDATION IN LIST

Question

i am  new to SharePoint

Currently on a project management project .

i have 2 list

    1 project detail list

    2  task list

i have a start date and due date for the parent project .i have a start date and due date for the task list of the project.
the problem is when i select the parent project name\id in the task list using the look-up.and obtain the start and due date of the parent project in that list .
i am not able to validate those fields as again the start date and due date of the the task list (as those columns are not visible in the list settings validation)


Could someone please help me how to get about this