How to write "paid in full" on check?

You may want to send a letter accompanying the check stating that by cashing the check the company agrees that you have paid a certain debt in full satisfaction of whatever it is you have satisfied. I would write "Account # - Paid in Full" clearly and legibly on the check where there is room, and it might be in the memo section. I don't think that there is any "legal" way to do it. Much would probably depend upon the creditor's ability to see and understand the comment and accept the terms. With the letter you avoid the issue of whether there was a lack of knowledge and understanding.