verify file before linking to another

I am wondering if there is a good tutorial or example of code on how to verify that a page has been hit before linking to the next. For example say you have a form, the form once all fields have been filled links to a confirmation page then from there it links to a thank you page which inserts data to a text file, how can one verify that someone is not just typing in the name of the last page and therefore inserting a blank entry into the text file? Any help would be great. Thanks in advance, jas