If an identity thief steals your paper checks or misuses the account number from the bottom of your checks, contact your bank or financial institution and ask them to close the account as soon as possible. Federal law doesn’t limit your loss if a thief forges your signature on your checks or uses your account number to buy something by phone, but most states hold banks responsible for losses from those fraudulent transactions. However, banks expect their customers to take reasonable care of their accounts. That means you might be responsible for a loss if you know about a problem but don’t report it to your bank quickly.
You should also ask your bank to report the theft to its check verification system which will tell businesses to refuse the stolen checks.