How can i kow for sure if my loan is goin to be paid?in a trade in with my car?

im trading my car in for another and i want to make sure my current car loan will be paid off? Couldnt i get it in writing that they agree to pay off my loan? Or is there anything i can do to make sure i dont want to be stuck with 2 payments? someone told me to get the loan number from the payments stubs on the new vechile but what do i do after that?