I need to echo the answers before the form is submitted so the user can check them, but I couldn't find any way of doing it through a script or app.
How can I do it?
In Google Forms, there is no built-in or Google Apps Script way to echo the answers before form submission.
One alternative is to create a custom form by using Google Apps Script.
Do like StackOverflow does it. Use the keystroke up event to fill an input or label with the values. On every event check for inputs that are filled and concatenate them in a pleasantly viewable way.