How can I check the user filled data of a task?

I suppose the only way is to query the DB, right? Could you point me to the correct table?

Could you clarify a little more?

For background, with a form submission, you could use client side JavaScript to validate and enforce data correctness and validity.
By default, on form submission, the form variables will typically be persisted as process variables. Hence you can access the variables as process variables inside say a service or script taskā€¦